Added to track the net balance and offset of the original entries.

src/accounting/models.py

@@ -21,6 +21,7 @@ from __future__ import annotations
 
 import re
 import typing as t
+from datetime import date
 from decimal import Decimal
 
 import sqlalchemy as sa
@@ -568,6 +569,21 @@ class Transaction(db.Model):
                 return False
         return True
 
+    @property
+    def can_delete(self) -> bool:
+        """Returns whether the transaction can be deleted.
+
+        :return: True if the transaction can be deleted, or False otherwise.
+        """
+        if not hasattr(self, "__can_delete"):
+            def has_offset() -> bool:
+                for entry in self.entries:
+                    if len(entry.offsets) > 0:
+                        return True
+                return False
+            setattr(self, "__can_delete", not has_offset())
+        return getattr(self, "__can_delete")
+
     def delete(self) -> None:
         """Deletes the transaction.
 
@@ -624,6 +640,21 @@ class JournalEntry(db.Model):
     amount = db.Column(db.Numeric(14, 2), nullable=False)
     """The amount."""
 
+    def __str__(self) -> str:
+        """Returns the string representation of the journal entry.
+
+        :return: The string representation of the journal entry.
+        """
+        if not hasattr(self, "__str"):
+            from accounting.template_filters import format_date, format_amount
+            setattr(self, "__str",
+                    gettext("%(date)s %(summary)s %(amount)s",
+                            date=format_date(self.transaction.date),
+                            summary="" if self.summary is None
+                            else self.summary,
+                            amount=format_amount(self.amount)))
+        return getattr(self, "__str")
+
     @property
     def eid(self) -> int | None:
         """Returns the journal entry ID.  This is the alternative name of the
@@ -649,6 +680,20 @@ class JournalEntry(db.Model):
         """
         return self.amount if self.is_debit else None
 
+    @property
+    def is_original_entry(self) -> bool:
+        """Returns whether the entry is an original entry.
+
+        :return: True if the entry is an original entry, or False otherwise.
+        """
+        if not self.account.is_offset_needed:
+            return False
+        if self.account.base_code[0] == "1" and not self.is_debit:
+            return False
+        if self.account.base_code[0] == "2" and self.is_debit:
+            return False
+        return True
+
     @property
     def credit(self) -> Decimal | None:
         """Returns the credit amount.
@@ -656,3 +701,45 @@ class JournalEntry(db.Model):
         :return: The credit amount, or None if this is not a credit entry.
         """
         return None if self.is_debit else self.amount
+
+    @property
+    def net_balance(self) -> Decimal:
+        """Returns the net balance.
+
+        :return: The net balance.
+        """
+        if not hasattr(self, "__net_balance"):
+            setattr(self, "__net_balance", self.amount + sum(
+                [x.amount if x.is_debit == self.is_debit else -x.amount
+                 for x in self.offsets]))
+        return getattr(self, "__net_balance")
+
+    @net_balance.setter
+    def net_balance(self, net_balance: Decimal) -> None:
+        """Sets the net balance.
+
+        :param net_balance: The net balance.
+        :return: None.
+        """
+        setattr(self, "__net_balance", net_balance)
+
+    @property
+    def query_values(self) -> tuple[list[str], list[str]]:
+        """Returns the values to be queried.
+
+        :return: The values to be queried.
+        """
+        def format_amount(value: Decimal) -> str:
+            whole: int = int(value)
+            frac: Decimal = (value - whole).normalize()
+            return str(whole) + str(abs(frac))[1:]
+
+        txn_day: date = self.transaction.date
+        summary: str = "" if self.summary is None else self.summary
+        return ([summary],
+                [str(txn_day.year),
+                 "{}/{}".format(txn_day.year, txn_day.month),
+                 "{}/{}".format(txn_day.month, txn_day.day),
+                 "{}/{}/{}".format(txn_day.year, txn_day.month, txn_day.day),
+                 format_amount(self.amount),
+                 format_amount(self.net_balance)])

src/accounting/static/css/style.css

@@ -31,6 +31,9 @@
     color: #141619;
     background-color: #D3D3D4;
 }
+.form-control.accounting-disabled {
+    background-color: #e9ecef;
+}
 
 /** The toolbar */
 .accounting-toolbar {
@@ -113,6 +116,33 @@
     border-bottom: thick double slategray;
 }
 
+/* Links between objects */
+.accounting-original-entry {
+    border-top: thin solid darkslategray;
+    padding: 0.2rem 0.5rem;
+}
+.accounting-original-entry a {
+    color: inherit;
+    text-decoration: none;
+}
+.accounting-original-entry a:hover {
+    color: inherit;
+}
+.accounting-offset-entries {
+    border-top: thin solid darkslategray;
+    padding: 0.2rem 0.5rem;
+}
+.accounting-offset-entries ul li {
+    list-style: none;
+}
+.accounting-offset-entries ul li a {
+    color: inherit;
+    text-decoration: none;
+}
+.accounting-offset-entries ul li a:hover {
+    color: inherit;
+}
+
 /** The option selector */
 .accounting-selector-list {
     height: 20rem;
@@ -150,6 +180,9 @@
     font-weight: bolder;
     border-top: thick double slategray;
 }
+.accounting-entry-editor-original-entry-content {
+    width: calc(100% - 3rem);
+}
 
 /* The report table */
 .accounting-report-table-header, .accounting-report-table-footer {

src/accounting/static/js/account-selector.js

@@ -111,7 +111,7 @@ class AccountSelector {
         };
         for (const option of this.#options) {
             option.onclick = () => {
-                this.#entryEditor.saveAccount(option.dataset.code, option.dataset.content);
+                this.#entryEditor.saveAccount(option.dataset.code, option.dataset.content, option.classList.contains("accounting-account-is-offset-needed"));
             };
         }
         this.#query.addEventListener("input", () => {

src/accounting/static/js/journal-entry-editor.js

@@ -57,6 +57,36 @@ class JournalEntryEditor {
      */
     #prefix = "accounting-entry-editor"
 
+    /**
+     * The container of the original entry
+     * @type {HTMLDivElement}
+     */
+    #originalEntryContainer;
+
+    /**
+     * The control of the original entry
+     * @type {HTMLDivElement}
+     */
+    #originalEntryControl;
+
+    /**
+     * The original entry
+     * @type {HTMLDivElement}
+     */
+    #originalEntry;
+
+    /**
+     * The error message of the original entry
+     * @type {HTMLDivElement}
+     */
+    #originalEntryError;
+
+    /**
+     * The delete button of the original entry
+     * @type {HTMLButtonElement}
+     */
+    #originalEntryDelete;
+
     /**
      * The control of the summary
      * @type {HTMLDivElement}
@@ -109,7 +139,7 @@ class JournalEntryEditor {
      * The journal entry to edit
      * @type {JournalEntrySubForm|null}
      */
-    #entry;
+    entry;
 
     /**
      * The debit or credit entry side sub-form
@@ -124,6 +154,11 @@ class JournalEntryEditor {
     constructor() {
         this.#element = document.getElementById(this.#prefix);
         this.#modal = document.getElementById(this.#prefix + "-modal");
+        this.#originalEntryContainer = document.getElementById(this.#prefix + "-original-entry-container");
+        this.#originalEntryControl = document.getElementById(this.#prefix + "-original-entry-control");
+        this.#originalEntry = document.getElementById(this.#prefix + "-original-entry");
+        this.#originalEntryError = document.getElementById(this.#prefix + "-original-entry-error");
+        this.#originalEntryDelete = document.getElementById(this.#prefix + "-original-entry-delete");
         this.#summaryControl = document.getElementById(this.#prefix + "-summary-control");
         this.#summary = document.getElementById(this.#prefix + "-summary");
         this.#summaryError = document.getElementById(this.#prefix + "-summary-error");
@@ -131,25 +166,90 @@ class JournalEntryEditor {
         this.#account = document.getElementById(this.#prefix + "-account");
         this.#accountError = document.getElementById(this.#prefix + "-account-error")
         this.#amount = document.getElementById(this.#prefix + "-amount");
-        this.#amountError = document.getElementById(this.#prefix + "-amount-error")
+        this.#amountError = document.getElementById(this.#prefix + "-amount-error");
+        this.#originalEntryControl.onclick = () => OriginalEntrySelector.start(this, this.#originalEntry.dataset.id);
+        this.#originalEntryDelete.onclick = () => this.clearOriginalEntry();
         this.#summaryControl.onclick = () => {
             SummaryEditor.start(this, this.#summary.dataset.value);
         };
         this.#accountControl.onclick = () => {
             AccountSelector.start(this, this.entryType);
         }
+        this.#amount.onchange = () => {
+            this.#validateAmount();
+        }
         this.#element.onsubmit = () => {
             if (this.#validate()) {
-                if (this.#entry === null) {
+                if (this.entry === null) {
-                    this.#entry = this.#side.addJournalEntry();
+                    this.entry = this.#side.addJournalEntry();
                 }
-                this.#entry.save(this.#account.dataset.code, this.#account.dataset.text, this.#summary.dataset.value, this.#amount.value);
+                this.entry.save("isOriginalEntry" in this.#element.dataset,this.#originalEntry.dataset.id, this.#originalEntry.dataset.date, this.#originalEntry.dataset.text, this.#account.dataset.code, this.#account.dataset.text, this.#summary.dataset.value, this.#amount.value);
                 bootstrap.Modal.getInstance(this.#modal).hide();
             }
             return false;
         };
     }
 
+    /**
+     * Saves the original entry from the original entry selector.
+     *
+     * @param originalEntry {OriginalEntry} the original entry
+     */
+    saveOriginalEntry(originalEntry) {
+        delete this.#element.dataset.isOriginalEntry;
+        this.#originalEntryContainer.classList.remove("d-none");
+        this.#originalEntryControl.classList.add("accounting-not-empty");
+        this.#originalEntry.dataset.id = originalEntry.id;
+        this.#originalEntry.dataset.date = originalEntry.date;
+        this.#originalEntry.dataset.text = originalEntry.text;
+        this.#originalEntry.innerText = originalEntry.text;
+        this.#setEnableSummaryAccount(false);
+        if (originalEntry.summary === "") {
+            this.#summaryControl.classList.remove("accounting-not-empty");
+        } else {
+            this.#summaryControl.classList.add("accounting-not-empty");
+        }
+        this.#summary.dataset.value = originalEntry.summary;
+        this.#summary.innerText = originalEntry.summary;
+        this.#accountControl.classList.add("accounting-not-empty");
+        this.#account.dataset.code = originalEntry.accountCode;
+        this.#account.dataset.text = originalEntry.accountText;
+        this.#account.innerText = originalEntry.accountText;
+        this.#amount.value = String(originalEntry.netBalance);
+        this.#amount.max = String(originalEntry.netBalance);
+        this.#amount.min = "0";
+        this.#validate();
+    }
+
+    /**
+     * Clears the original entry.
+     *
+     */
+    clearOriginalEntry() {
+        delete this.#element.dataset.isOriginalEntry;
+        this.#originalEntryContainer.classList.add("d-none");
+        this.#originalEntryControl.classList.remove("accounting-not-empty");
+        this.#originalEntry.dataset.id = "";
+        this.#originalEntry.dataset.date = "";
+        this.#originalEntry.dataset.text = "";
+        this.#originalEntry.innerText = "";
+        this.#setEnableSummaryAccount(true);
+        this.#accountControl.classList.remove("accounting-not-empty");
+        this.#account.dataset.code = "";
+        this.#account.dataset.text = "";
+        this.#account.innerText = "";
+        this.#amount.max = "";
+    }
+
+    /**
+     * Returns the currency code.
+     *
+     * @return {string} the currency code
+     */
+    getCurrencyCode() {
+        return this.#side.currency.getCurrencyCode();
+    }
+
     /**
      * Returns the transaction form.
      *
@@ -172,6 +272,7 @@ class JournalEntryEditor {
         }
         this.#summary.dataset.value = summary;
         this.#summary.innerText = summary;
+        this.#validateSummary();
         bootstrap.Modal.getOrCreateInstance(this.#modal).show();
     }
 
@@ -181,8 +282,14 @@ class JournalEntryEditor {
      * @param summary {string} the summary
      * @param accountCode {string} the account code
      * @param accountText {string} the account text
+     * @param isAccountOffsetNeeded {boolean} true if the journal entries in the account need offset, or false otherwise
      */
-    saveSummaryWithAccount(summary, accountCode, accountText) {
+    saveSummaryWithAccount(summary, accountCode, accountText, isAccountOffsetNeeded) {
+        if (isAccountOffsetNeeded) {
+            this.#element.dataset.isOriginalEntry = "true";
+        } else {
+            delete this.#element.dataset.isOriginalEntry;
+        }
         this.#accountControl.classList.add("accounting-not-empty");
         this.#account.dataset.code = accountCode;
         this.#account.dataset.text = accountText;
@@ -205,6 +312,7 @@ class JournalEntryEditor {
      *
      */
     clearAccount() {
+        delete this.#element.dataset.isOriginalEntry;
         this.#accountControl.classList.remove("accounting-not-empty");
         this.#account.dataset.code = "";
         this.#account.dataset.text = "";
@@ -217,8 +325,14 @@ class JournalEntryEditor {
      *
      * @param code {string} the account code
      * @param text {string} the account text
+     * @param isOffsetNeeded {boolean} true if the journal entries in the account need offset or false otherwise
      */
-    saveAccount(code, text) {
+    saveAccount(code, text, isOffsetNeeded) {
+        if (isOffsetNeeded) {
+            this.#element.dataset.isOriginalEntry = "true";
+        } else {
+            delete this.#element.dataset.isOriginalEntry;
+        }
         this.#accountControl.classList.add("accounting-not-empty");
         this.#account.dataset.code = code;
         this.#account.dataset.text = text;
@@ -233,12 +347,25 @@ class JournalEntryEditor {
      */
     #validate() {
         let isValid = true;
+        isValid = this.#validateOriginalEntry() && isValid;
         isValid = this.#validateSummary() && isValid;
         isValid = this.#validateAccount() && isValid;
         isValid = this.#validateAmount() && isValid
         return isValid;
     }
 
+    /**
+     * Validates the original entry.
+     *
+     * @return {boolean} true if valid, or false otherwise
+     * @private
+     */
+    #validateOriginalEntry() {
+        this.#originalEntryControl.classList.remove("is-invalid");
+        this.#originalEntryError.innerText = "";
+        return true;
+    }
+
     /**
      * Validates the summary.
      *
@@ -281,8 +408,29 @@ class JournalEntryEditor {
             this.#amountError.innerText = A_("Please fill in the amount.");
             return false;
         }
+        const amount =new Decimal(this.#amount.value);
+        if (amount.lessThanOrEqualTo(0)) {
+            this.#amount.classList.add("is-invalid");
+            this.#amountError.innerText = A_("Please fill in a positive amount.");
+            return false;
+        }
+        if (this.#amount.max !== "") {
+            if (amount.greaterThan(new Decimal(this.#amount.max))) {
+                this.#amount.classList.add("is-invalid");
+                this.#amountError.innerText = A_("The amount must not exceed the net balance %(balance)s of the original entry.", {balance: new Decimal(this.#amount.max)});
+                return false;
+            }
+        }
+        if (this.#amount.min !== "") {
+            const min = new Decimal(this.#amount.min);
+            if (amount.lessThan(min)) {
+                this.#amount.classList.add("is-invalid");
+                this.#amountError.innerText = A_("The amount must not be less than the offset total %(total)s.", {total: formatDecimal(min)});
+                return false;
+            }
+        }
         this.#amount.classList.remove("is-invalid");
-        this.#amount.innerText = "";
+        this.#amountError.innerText = "";
         return true;
     }
 
@@ -292,24 +440,33 @@ class JournalEntryEditor {
      * @param side {DebitCreditSideSubForm} the debit or credit side sub-form
      */
     #onAddNew(side) {
-        this.#entry = null;
+        this.entry = null;
         this.#side = side;
         this.entryType = this.#side.entryType;
         this.#element.dataset.entryType = side.entryType;
-        this.#summaryControl.dataset.bsTarget = "#accounting-summary-editor-" + side.entryType + "-modal";
+        delete this.#element.dataset.isOriginalEntry;
+        this.#originalEntryContainer.classList.add("d-none");
+        this.#originalEntryControl.classList.remove("accounting-not-empty");
+        this.#originalEntryControl.classList.remove("is-invalid");
+        this.#originalEntry.dataset.id = "";
+        this.#originalEntry.dataset.date = "";
+        this.#originalEntry.dataset.text = "";
+        this.#originalEntry.innerText = "";
+        this.#setEnableSummaryAccount(true);
         this.#summaryControl.classList.remove("accounting-not-empty");
         this.#summaryControl.classList.remove("is-invalid");
         this.#summary.dataset.value = "";
         this.#summary.innerText = ""
         this.#summaryError.innerText = ""
-        this.#accountControl.dataset.bsTarget = "#accounting-account-selector-" + side.entryType + "-modal";
         this.#accountControl.classList.remove("accounting-not-empty");
         this.#accountControl.classList.remove("is-invalid");
-        this.#account.innerText = "";
         this.#account.dataset.code = "";
         this.#account.dataset.text = "";
+        this.#account.innerText = "";
         this.#accountError.innerText = "";
         this.#amount.value = "";
+        this.#amount.max = "";
+        this.#amount.min = "0";
         this.#amount.classList.remove("is-invalid");
         this.#amountError.innerText = "";
     }
@@ -318,17 +475,37 @@ class JournalEntryEditor {
      * Edits a journal entry.
      *
      * @param entry {JournalEntrySubForm} the journal entry sub-form
+     * @param originalEntryId {string} the ID of the original entry
+     * @param originalEntryDate {string} the date of the original entry
+     * @param originalEntryText {string} the text of the original entry
      * @param summary {string} the summary
      * @param accountCode {string} the account code
      * @param accountText {string} the account text
      * @param amount {string} the amount
+     * @param amountMin {string} the minimal amount
      */
-    #onEdit(entry, summary, accountCode, accountText, amount) {
+    #onEdit(entry, originalEntryId, originalEntryDate, originalEntryText, summary, accountCode, accountText, amount, amountMin) {
-        this.#entry = entry;
+        this.entry = entry;
         this.#side = entry.side;
         this.entryType = this.#side.entryType;
         this.#element.dataset.entryType = entry.entryType;
-        this.#summaryControl.dataset.bsTarget = "#accounting-summary-editor-" + entry.entryType + "-modal";
+        if (entry.isOriginalEntry()) {
+            this.#element.dataset.isOriginalEntry = "true";
+        } else {
+            delete this.#element.dataset.isOriginalEntry;
+        }
+        if (originalEntryId === "") {
+            this.#originalEntryContainer.classList.add("d-none");
+            this.#originalEntryControl.classList.remove("accounting-not-empty");
+        } else {
+            this.#originalEntryContainer.classList.remove("d-none");
+            this.#originalEntryControl.classList.add("accounting-not-empty");
+        }
+        this.#originalEntry.dataset.id = originalEntryId;
+        this.#originalEntry.dataset.date = originalEntryDate;
+        this.#originalEntry.dataset.text = originalEntryText;
+        this.#originalEntry.innerText = originalEntryText;
+        this.#setEnableSummaryAccount(!entry.isMatched && originalEntryId === "");
         if (summary === "") {
             this.#summaryControl.classList.remove("accounting-not-empty");
         } else {
@@ -336,16 +513,58 @@ class JournalEntryEditor {
         }
         this.#summary.dataset.value = summary;
         this.#summary.innerText = summary;
-        this.#accountControl.dataset.bsTarget = "#accounting-account-selector-" + entry.entryType + "-modal";
         if (accountCode === "") {
             this.#accountControl.classList.remove("accounting-not-empty");
         } else {
             this.#accountControl.classList.add("accounting-not-empty");
         }
-        this.#account.innerText = accountText;
         this.#account.dataset.code = accountCode;
         this.#account.dataset.text = accountText;
+        this.#account.innerText = accountText;
         this.#amount.value = amount;
+        const maxAmount = this.#getMaxAmount();
+        this.#amount.max = maxAmount === null? "": maxAmount;
+        this.#amount.min = amountMin;
+        this.#validate();
+    }
+
+    /**
+     * Finds out the max amount.
+     *
+     * @return {Decimal|null} the max amount
+     */
+    #getMaxAmount() {
+        if (this.#originalEntry.dataset.id === "") {
+            return null;
+        }
+        return OriginalEntrySelector.getNetBalance(this.entry, this.getTransactionForm(), this.#originalEntry.dataset.id);
+    }
+
+    /**
+     * Sets the enable status of the summary and account.
+     *
+     * @param isEnabled {boolean} true to enable, or false otherwise
+     */
+    #setEnableSummaryAccount(isEnabled) {
+        if (isEnabled) {
+            this.#summaryControl.dataset.bsToggle = "modal";
+            this.#summaryControl.dataset.bsTarget = "#accounting-summary-editor-" + this.#side.entryType + "-modal";
+            this.#summaryControl.classList.remove("accounting-disabled");
+            this.#summaryControl.classList.add("accounting-clickable");
+            this.#accountControl.dataset.bsToggle = "modal";
+            this.#accountControl.dataset.bsTarget = "#accounting-account-selector-" + this.#side.entryType + "-modal";
+            this.#accountControl.classList.remove("accounting-disabled");
+            this.#accountControl.classList.add("accounting-clickable");
+        } else {
+            this.#summaryControl.dataset.bsToggle = "";
+            this.#summaryControl.dataset.bsTarget = "";
+            this.#summaryControl.classList.add("accounting-disabled");
+            this.#summaryControl.classList.remove("accounting-clickable");
+            this.#accountControl.dataset.bsToggle = "";
+            this.#accountControl.dataset.bsTarget = "";
+            this.#accountControl.classList.add("accounting-disabled");
+            this.#accountControl.classList.remove("accounting-clickable");
+        }
     }
 
     /**
@@ -375,12 +594,16 @@ class JournalEntryEditor {
      * Edits a journal entry.
      *
      * @param entry {JournalEntrySubForm} the journal entry sub-form
+     * @param originalEntryId {string} the ID of the original entry
+     * @param originalEntryDate {string} the date of the original entry
+     * @param originalEntryText {string} the text of the original entry
      * @param summary {string} the summary
      * @param accountCode {string} the account code
      * @param accountText {string} the account text
      * @param amount {string} the amount
+     * @param amountMin {string} the minimal amount
      */
-    static edit(entry, summary, accountCode, accountText, amount) {
+    static edit(entry, originalEntryId, originalEntryDate, originalEntryText, summary, accountCode, accountText, amount, amountMin) {
-        this.#editor.#onEdit(entry, summary, accountCode, accountText, amount);
+        this.#editor.#onEdit(entry, originalEntryId, originalEntryDate, originalEntryText, summary, accountCode, accountText, amount, amountMin);
     }
 }

src/accounting/static/js/original-entry-selector.js

@@ -0,0 +1,447 @@
+/* The Mia! Accounting Flask Project
+ * original-entry-selector.js: The JavaScript for the original entry selector
+ */
+
+/*  Copyright (c) 2023 imacat.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+/* Author: imacat@mail.imacat.idv.tw (imacat)
+ * First written: 2023/3/10
+ */
+"use strict";
+
+// Initializes the page JavaScript.
+document.addEventListener("DOMContentLoaded", () => {
+    OriginalEntrySelector.initialize();
+});
+
+/**
+ * The original entry selector.
+ *
+ */
+class OriginalEntrySelector {
+
+    /**
+     * The prefix of the HTML ID and class
+     * @type {string}
+     */
+    #prefix = "accounting-original-entry-selector";
+
+    /**
+     * The modal of the original entry editor
+     * @type {HTMLDivElement}
+     */
+    #modal;
+
+    /**
+     * The query input
+     * @type {HTMLInputElement}
+     */
+    #query;
+
+    /**
+     * The error message when the query has no result
+     * @type {HTMLParagraphElement}
+     */
+    #queryNoResult;
+
+    /**
+     * The option list
+     * @type {HTMLUListElement}
+     */
+    #optionList;
+
+    /**
+     * The options
+     * @type {OriginalEntry[]}
+     */
+    #options;
+
+    /**
+     * The options by their ID
+     * @type {Object.<string, OriginalEntry>}
+     */
+    #optionById;
+
+    /**
+     * The journal entry editor.
+     * @type {JournalEntryEditor}
+     */
+    entryEditor;
+
+    /**
+     * Constructs an original entry selector.
+     *
+     */
+    constructor() {
+        this.#modal = document.getElementById(this.#prefix + "-modal");
+        this.#query = document.getElementById(this.#prefix + "-query");
+        this.#queryNoResult = document.getElementById(this.#prefix + "-option-no-result");
+        this.#optionList = document.getElementById(this.#prefix + "-option-list");
+        this.#options = Array.from(document.getElementsByClassName(this.#prefix + "-option")).map((element) => new OriginalEntry(this, element));
+        this.#optionById = {};
+        for (const option of this.#options) {
+            this.#optionById[option.id] = option;
+        }
+        this.#query.addEventListener("input", () => {
+            this.#filterOptions();
+        });
+    }
+
+    /**
+     * Returns the net balance for an original entry.
+     *
+     * @param currentEntry {JournalEntrySubForm} the journal entry sub-form that is currently editing
+     * @param form {TransactionForm} the transaction form
+     * @param originalEntryId {string} the ID of the original entry
+     * @return {Decimal} the net balance of the original entry
+     */
+    #getNetBalance(currentEntry, form, originalEntryId) {
+        const otherEntries = form.getEntries().filter((entry) => entry !== currentEntry);
+        let otherOffset = new Decimal(0);
+        for (const otherEntry of otherEntries) {
+            if (otherEntry.getOriginalEntryId() === originalEntryId) {
+                const amount = otherEntry.getAmount();
+                if (amount !== null) {
+                    otherOffset = otherOffset.plus(amount);
+                }
+            }
+        }
+        return this.#optionById[originalEntryId].bareNetBalance.minus(otherOffset);
+    }
+
+    /**
+     * Updates the net balances, subtracting the offset amounts on the form but the currently editing journal entry
+     *
+     */
+    #updateNetBalances() {
+        const otherEntries = this.entryEditor.getTransactionForm().getEntries().filter((entry) => entry !== this.entryEditor.entry);
+        const otherOffsets = {}
+        for (const otherEntry of otherEntries) {
+            const otherOriginalEntryId = otherEntry.getOriginalEntryId();
+            const amount = otherEntry.getAmount();
+            if (otherOriginalEntryId === null || amount === null) {
+                continue;
+            }
+            if (!(otherOriginalEntryId in otherOffsets)) {
+                otherOffsets[otherOriginalEntryId] = new Decimal("0");
+            }
+            otherOffsets[otherOriginalEntryId] = otherOffsets[otherOriginalEntryId].plus(amount);
+        }
+        for (const option of this.#options) {
+            if (option.id in otherOffsets) {
+                option.updateNetBalance(otherOffsets[option.id]);
+            } else {
+                option.resetNetBalance();
+            }
+        }
+    }
+
+    /**
+     * Filters the options.
+     *
+     */
+    #filterOptions() {
+        let hasAnyMatched = false;
+        for (const option of this.#options) {
+            if (option.isMatched(this.#modal.dataset.entryType, this.#modal.dataset.currencyCode, this.#query.value)) {
+                option.setShown(true);
+                hasAnyMatched = true;
+            } else {
+                option.setShown(false);
+            }
+        }
+        if (!hasAnyMatched) {
+            this.#optionList.classList.add("d-none");
+            this.#queryNoResult.classList.remove("d-none");
+        } else {
+            this.#optionList.classList.remove("d-none");
+            this.#queryNoResult.classList.add("d-none");
+        }
+    }
+
+    /**
+     * The callback when the original entry selector is shown.
+     *
+     * @param entryEditor {JournalEntryEditor} the journal entry editor
+     * @param originalEntryId {string|null} the ID of the original entry
+     */
+    #onOpen(entryEditor, originalEntryId) {
+        this.entryEditor = entryEditor
+        this.#modal.dataset.currencyCode = entryEditor.getCurrencyCode();
+        this.#modal.dataset.entryType = entryEditor.entryType;
+        for (const option of this.#options) {
+            option.setActive(option.id === originalEntryId);
+        }
+        this.#query.value = "";
+        this.#updateNetBalances();
+        this.#filterOptions();
+    }
+
+    /**
+     * The original entry selector.
+     * @type {OriginalEntrySelector}
+     */
+    static #selector;
+
+    /**
+     * Initializes the original entry selector.
+     *
+     */
+    static initialize() {
+        this.#selector = new OriginalEntrySelector();
+    }
+
+    /**
+     * Starts the original entry selector.
+     *
+     * @param entryEditor {JournalEntryEditor} the journal entry editor
+     * @param originalEntryId {string|null} the ID of the original entry
+     */
+    static start(entryEditor, originalEntryId = null) {
+        this.#selector.#onOpen(entryEditor, originalEntryId);
+    }
+
+    /**
+     * Returns the net balance for an original entry.
+     *
+     * @param currentEntry {JournalEntrySubForm} the journal entry sub-form that is currently editing
+     * @param form {TransactionForm} the transaction form
+     * @param originalEntryId {string} the ID of the original entry
+     * @return {Decimal} the net balance of the original entry
+     */
+    static getNetBalance(currentEntry, form, originalEntryId) {
+        return this.#selector.#getNetBalance(currentEntry, form, originalEntryId);
+    }
+}
+
+/**
+ * An original entry.
+ *
+ */
+class OriginalEntry {
+
+    /**
+     * The original entry selector
+     * @type {OriginalEntrySelector}
+     */
+    #selector;
+
+    /**
+     * The element
+     * @type {HTMLLIElement}
+     */
+    #element;
+
+    /**
+     * The ID
+     * @type {string}
+     */
+    id;
+
+    /**
+     * The date
+     * @type {string}
+     */
+    date;
+
+    /**
+     * The entry type, either "debit" or "credit"
+     * @type {string}
+     */
+    #entryType;
+
+    /**
+     * The currency code
+     * @type {string}
+     */
+    #currencyCode;
+
+    /**
+     * The account code
+     * @type {string}
+     */
+    accountCode;
+
+    /**
+     * The account text
+     * @type {string}
+     */
+    accountText;
+
+    /**
+     * The summary
+     * @type {string}
+     */
+    summary;
+
+    /**
+     * The net balance, without the offset amounts on the form
+     * @type {Decimal}
+     */
+    bareNetBalance;
+
+    /**
+     * The net balance
+     * @type {Decimal}
+     */
+    netBalance;
+
+    /**
+     * The text of the net balance
+     * @type {HTMLSpanElement}
+     */
+    netBalanceText;
+
+    /**
+     * The text representation of the original entry
+     * @type {string}
+     */
+    text;
+
+    /**
+     * The values to query against
+     * @type {string[][]}
+     */
+    #queryValues;
+
+    /**
+     * Constructs an original entry.
+     *
+     * @param selector {OriginalEntrySelector} the original entry selector
+     * @param element {HTMLLIElement} the element
+     */
+    constructor(selector, element) {
+        this.#selector = selector;
+        this.#element = element;
+        this.id = element.dataset.id;
+        this.date = element.dataset.date;
+        this.#entryType = element.dataset.entryType;
+        this.#currencyCode = element.dataset.currencyCode;
+        this.accountCode = element.dataset.accountCode;
+        this.accountText = element.dataset.accountText;
+        this.summary = element.dataset.summary;
+        this.bareNetBalance = new Decimal(element.dataset.netBalance);
+        this.netBalance = this.bareNetBalance;
+        this.netBalanceText = document.getElementById("accounting-original-entry-selector-option-" + this.id + "-net-balance");
+        this.text = element.dataset.text;
+        this.#queryValues = JSON.parse(element.dataset.queryValues);
+        this.#element.onclick = () => this.#selector.entryEditor.saveOriginalEntry(this);
+    }
+
+    /**
+     * Resets the net balance to its initial value, without the offset amounts on the form.
+     *
+     */
+    resetNetBalance() {
+        if (this.netBalance !== this.bareNetBalance) {
+            this.netBalance = this.bareNetBalance;
+            this.#updateNetBalanceText();
+        }
+    }
+
+    /**
+     * Updates the net balance with an offset.
+     *
+     * @param offset {Decimal} the offset to be added to the net balance
+     */
+    updateNetBalance(offset) {
+        this.netBalance = this.bareNetBalance.minus(offset);
+        this.#updateNetBalanceText();
+    }
+
+    /**
+     * Updates the text display of the net balance.
+     *
+     */
+    #updateNetBalanceText() {
+        this.netBalanceText.innerText = formatDecimal(this.netBalance);
+    }
+
+    /**
+     * Returns whether the original matches.
+     *
+     * @param entryType {string} the entry type, either "debit" or "credit"
+     * @param currencyCode {string} the currency code
+     * @param query {string|null} the query term
+     */
+    isMatched(entryType, currencyCode, query = null) {
+        return this.netBalance.greaterThan(0)
+            && this.date <= this.#selector.entryEditor.getTransactionForm().getDate()
+            && this.#isEntryTypeMatches(entryType)
+            && this.#currencyCode === currencyCode
+            && this.#isQueryMatches(query);
+    }
+
+    /**
+     * Returns whether the original entry matches the entry type.
+     *
+     * @param entryType {string} the entry type, either "debit" or credit
+     * @return {boolean} true if the option matches, or false otherwise
+     */
+    #isEntryTypeMatches(entryType) {
+        return (entryType === "debit" && this.#entryType === "credit")
+            || (entryType === "credit" && this.#entryType === "debit");
+    }
+
+    /**
+     * Returns whether the original entry matches the query.
+     *
+     * @param query {string|null} the query term
+     * @return {boolean} true if the option matches, or false otherwise
+     */
+    #isQueryMatches(query) {
+        if (query === "") {
+            return true;
+        }
+        for (const queryValue of this.#queryValues[0]) {
+            if (queryValue.toLowerCase().includes(query.toLowerCase())) {
+                return true;
+            }
+        }
+        for (const queryValue of this.#queryValues[1]) {
+            if (queryValue === query) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Sets whether the option is shown.
+     *
+     * @param isShown {boolean} true to show, or false otherwise
+     */
+    setShown(isShown) {
+        if (isShown) {
+            this.#element.classList.remove("d-none");
+        } else {
+            this.#element.classList.add("d-none");
+        }
+    }
+
+    /**
+     * Sets whether the option is active.
+     *
+     * @param isActive {boolean} true if active, or false otherwise
+     */
+    setActive(isActive) {
+        if (isActive) {
+            this.#element.classList.add("active");
+        } else {
+            this.#element.classList.remove("active");
+        }
+    }
+}

src/accounting/static/js/summary-editor.js

@@ -69,6 +69,12 @@ class SummaryEditor {
      */
     summary;
 
+    /**
+     * The button to the original entry selector
+     * @type {HTMLButtonElement}
+     */
+    #offsetButton;
+
     /**
      * The number input
      * @type {HTMLInputElement}
@@ -116,6 +122,7 @@ class SummaryEditor {
         this.prefix = "accounting-summary-editor-" + form.dataset.entryType;
         this.#modal = document.getElementById(this.prefix + "-modal");
         this.summary = document.getElementById(this.prefix + "-summary");
+        this.#offsetButton = document.getElementById(this.prefix + "-offset");
         this.number = document.getElementById(this.prefix + "-annotation-number");
         this.note = document.getElementById(this.prefix + "-annotation-note");
         // noinspection JSValidateTypes
@@ -128,6 +135,7 @@ class SummaryEditor {
         this.currentTab = this.tabPlanes.general;
         this.#initializeSuggestedAccounts();
         this.summary.onchange = () => this.#onSummaryChange();
+        this.#offsetButton.onclick = () => OriginalEntrySelector.start(this.#entryEditor);
         this.#form.onsubmit = () => {
             if (this.currentTab.validate()) {
                 this.#submit();
@@ -210,7 +218,7 @@ class SummaryEditor {
     #submit() {
         bootstrap.Modal.getOrCreateInstance(this.#modal).hide();
         if (this.#selectedAccount !== null) {
-            this.#entryEditor.saveSummaryWithAccount(this.summary.value, this.#selectedAccount.dataset.code, this.#selectedAccount.dataset.text);
+            this.#entryEditor.saveSummaryWithAccount(this.summary.value, this.#selectedAccount.dataset.code, this.#selectedAccount.dataset.text, this.#selectedAccount.classList.contains("accounting-account-is-offset-needed"));
         } else {
             this.#entryEditor.saveSummary(this.summary.value);
         }

src/accounting/static/js/transaction-form.js

@@ -159,10 +159,22 @@ class TransactionForm {
             this.#currencies[0].deleteButton.classList.add("d-none");
         } else {
             for (const currency of this.#currencies) {
+                const isAnyEntryMatched = () => {
+                    for (const entry of currency.getEntries()) {
+                        if (entry.isMatched) {
+                            return true;
+                        }
+                    }
+                    return false;
+                };
+                if (isAnyEntryMatched()) {
+                    currency.deleteButton.classList.add("d-none");
+                } else {
                     currency.deleteButton.classList.remove("d-none");
                 }
             }
         }
+    }
 
     /**
      * Initializes the drag and drop reordering on the currency sub-forms.
@@ -178,6 +190,20 @@ class TransactionForm {
         });
     }
 
+    /**
+     * Returns all the journal entries in the form.
+     *
+     * @param entryType {string|null} the entry type, either "debit" or "credit", or null for both
+     * @return {JournalEntrySubForm[]} all the journal entry sub-forms
+     */
+    getEntries(entryType = null) {
+        const entries = [];
+        for (const currency of this.#currencies) {
+            entries.push(...currency.getEntries(entryType));
+        }
+        return entries;
+    }
+
     /**
      * Returns the account codes used in the form.
      *
@@ -185,11 +211,35 @@ class TransactionForm {
      * @return {string[]} the account codes used in the form
      */
     getAccountCodesUsed(entryType) {
-        let inUse = [];
+        return this.getEntries(entryType).map((entry) => entry.getAccountCode())
-        for (const currency of this.#currencies) {
+            .filter((code) => code !== null);
-            inUse = inUse.concat(currency.getAccountCodesUsed(entryType));
     }
-        return inUse;
+
+    /**
+     * Returns the date.
+     *
+     * @return {string} the date
+     */
+    getDate() {
+        return this.#date.value;
+    }
+
+    /**
+     * Updates the minimal date.
+     *
+     */
+    updateMinDate() {
+        let lastOriginalEntryDate = null;
+        for (const entry of this.getEntries()) {
+            const date = entry.getOriginalEntryDate();
+            if (date !== null) {
+                if (lastOriginalEntryDate === null || lastOriginalEntryDate < date) {
+                    lastOriginalEntryDate = date;
+                }
+            }
+        }
+        this.#date.min = lastOriginalEntryDate === null? "": lastOriginalEntryDate;
+        this.#validateDate();
     }
 
     /**
@@ -218,6 +268,11 @@ class TransactionForm {
             this.#dateError.innerText = A_("Please fill in the date.");
             return false;
         }
+        if (this.#date.value < this.#date.min) {
+            this.#date.classList.add("is-invalid");
+            this.#dateError.innerText = A_("The date cannot be earlier than the original entries.");
+            return false;
+        }
         this.#date.classList.remove("is-invalid");
         this.#dateError.innerText = "";
         return true;
@@ -330,6 +385,18 @@ class CurrencySubForm {
      */
     no;
 
+    /**
+     * The currency code
+     * @type {HTMLInputElement}
+     */
+    #code;
+
+    /**
+     * The currency code selector
+     * @type {HTMLSelectElement}
+     */
+    #codeSelect;
+
     /**
      * The button to delete the currency
      * @type {HTMLButtonElement}
@@ -362,11 +429,16 @@ class CurrencySubForm {
         this.#control = document.getElementById(this.#prefix + "-control");
         this.#error = document.getElementById(this.#prefix + "-error");
         this.no = document.getElementById(this.#prefix + "-no");
+        this.#code = document.getElementById(this.#prefix + "-code");
+        this.#codeSelect = document.getElementById(this.#prefix + "-code-select");
         this.deleteButton = document.getElementById(this.#prefix + "-delete");
         const debitElement = document.getElementById(this.#prefix + "-debit");
         this.#debit = debitElement === null? null: new DebitCreditSideSubForm(this, debitElement, "debit");
         const creditElement = document.getElementById(this.#prefix + "-credit");
         this.#credit = creditElement == null? null: new DebitCreditSideSubForm(this, creditElement, "credit");
+        this.#codeSelect.onchange = () => {
+            this.#code.value = this.#codeSelect.value;
+        };
         this.deleteButton.onclick = () => {
             this.element.parentElement.removeChild(this.element);
             this.form.deleteCurrency(this);
@@ -374,18 +446,45 @@ class CurrencySubForm {
     }
 
     /**
-     * Returns the account codes used in the form.
+     * Returns the currency code.
      *
-     * @param entryType {string} the entry type, either "debit" or "credit"
+     * @return {string} the currency code
-     * @return {string[]} the account codes used in the form
      */
-    getAccountCodesUsed(entryType) {
+    getCurrencyCode() {
-        if (entryType === "debit") {
+        return this.#code.value;
-            return this.#debit.getAccountCodesUsed();
-        } else if (entryType === "credit") {
-            return this.#credit.getAccountCodesUsed();
     }
-        return []
+
+    /**
+     * Returns all the journal entries in the form.
+     *
+     * @param entryType {string|null} the entry type, either "debit" or "credit", or null for both
+     * @return {JournalEntrySubForm[]} all the journal entry sub-forms
+     */
+    getEntries(entryType = null) {
+        const entries = []
+        for (const side of [this.#debit, this.#credit]) {
+            if (side !== null ) {
+                if (entryType === null || side.entryType === entryType) {
+                    entries.push(...side.entries);
+                }
+            }
+        }
+        return entries;
+    }
+
+    /**
+     * Updates whether to enable the currency code selector
+     *
+     */
+    updateCodeSelectorStatus() {
+        let isEnabled = true;
+        for (const entry of this.getEntries()) {
+            if (entry.getOriginalEntryId() !== null) {
+                isEnabled = false;
+                break;
+            }
+        }
+        this.#codeSelect.disabled = !isEnabled;
     }
 
     /**
@@ -476,7 +575,7 @@ class DebitCreditSideSubForm {
      * The journal entry sub-forms
      * @type {JournalEntrySubForm[]}
      */
-    #entries;
+    entries;
 
     /**
      * The total
@@ -506,7 +605,7 @@ class DebitCreditSideSubForm {
         this.#error = document.getElementById(this.#prefix + "-error");
         this.#entryList = document.getElementById(this.#prefix + "-list");
         // noinspection JSValidateTypes
-        this.#entries = Array.from(document.getElementsByClassName(this.#prefix)).map((element) => new JournalEntrySubForm(this, element));
+        this.entries = Array.from(document.getElementsByClassName(this.#prefix)).map((element) => new JournalEntrySubForm(this, element));
         this.#total = document.getElementById(this.#prefix + "-total");
         this.#addEntryButton = document.getElementById(this.#prefix + "-add-entry");
         this.#addEntryButton.onclick = () => {
@@ -522,14 +621,14 @@ class DebitCreditSideSubForm {
      * @returns {JournalEntrySubForm} the newly-added journal entry sub-form
      */
     addJournalEntry() {
-        const newIndex = 1 + (this.#entries.length === 0? 0: Math.max(...this.#entries.map((entry) => entry.entryIndex)));
+        const newIndex = 1 + (this.entries.length === 0? 0: Math.max(...this.entries.map((entry) => entry.entryIndex)));
         const html = this.currency.form.entryTemplate
             .replaceAll("CURRENCY_INDEX", escapeHtml(String(this.#currencyIndex)))
             .replaceAll("ENTRY_TYPE", escapeHtml(this.entryType))
             .replaceAll("ENTRY_INDEX", escapeHtml(String(newIndex)));
         this.#entryList.insertAdjacentHTML("beforeend", html);
         const entry = new JournalEntrySubForm(this, document.getElementById(this.#prefix + "-" + String(newIndex)));
-        this.#entries.push(entry);
+        this.entries.push(entry);
         this.#resetDeleteJournalEntryButtons();
         this.#initializeDragAndDropReordering();
         this.validate();
@@ -542,9 +641,11 @@ class DebitCreditSideSubForm {
      * @param entry {JournalEntrySubForm}
      */
     deleteJournalEntry(entry) {
-        const index = this.#entries.indexOf(entry);
+        const index = this.entries.indexOf(entry);
-        this.#entries.splice(index, 1);
+        this.entries.splice(index, 1);
         this.updateTotal();
+        this.currency.updateCodeSelectorStatus();
+        this.currency.form.updateMinDate();
         this.#resetDeleteJournalEntryButtons();
     }
 
@@ -553,14 +654,18 @@ class DebitCreditSideSubForm {
      *
      */
     #resetDeleteJournalEntryButtons() {
-        if (this.#entries.length === 1) {
+        if (this.entries.length === 1) {
-            this.#entries[0].deleteButton.classList.add("d-none");
+            this.entries[0].deleteButton.classList.add("d-none");
+        } else {
+            for (const entry of this.entries) {
+                if (entry.isMatched) {
+                    entry.deleteButton.classList.add("d-none");
                 } else {
-            for (const entry of this.#entries) {
                     entry.deleteButton.classList.remove("d-none");
                 }
             }
         }
+    }
 
     /**
      * Returns the total amount.
@@ -569,9 +674,10 @@ class DebitCreditSideSubForm {
      */
     getTotal() {
         let total = new Decimal("0");
-        for (const entry of this.#entries) {
+        for (const entry of this.entries) {
-            if (entry.amount.value !== "") {
+            const amount = entry.getAmount();
-                total = total.plus(new Decimal(entry.amount.value));
+            if (amount !== null) {
+                total = total.plus(amount);
             }
         }
         return total;
@@ -582,17 +688,10 @@ class DebitCreditSideSubForm {
      *
      */
     updateTotal() {
-        let total = new Decimal("0");
-        for (const entry of this.#entries) {
-            if (entry.amount.value !== "") {
-                total = total.plus(new Decimal(entry.amount.value));
-            }
-        }
         this.#total.innerText = formatDecimal(this.getTotal());
         this.currency.validateBalance();
     }
 
-
     /**
      * Initializes the drag and drop reordering on the currency sub-forms.
      *
@@ -600,22 +699,13 @@ class DebitCreditSideSubForm {
     #initializeDragAndDropReordering() {
         initializeDragAndDropReordering(this.#entryList, () => {
             const entryId = Array.from(this.#entryList.children).map((entry) => entry.id);
-            this.#entries.sort((a, b) => entryId.indexOf(a.element.id) - entryId.indexOf(b.element.id));
+            this.entries.sort((a, b) => entryId.indexOf(a.element.id) - entryId.indexOf(b.element.id));
-            for (let i = 0; i < this.#entries.length; i++) {
+            for (let i = 0; i < this.entries.length; i++) {
-                this.#entries[i].no.value = String(i + 1);
+                this.entries[i].no.value = String(i + 1);
             }
         });
     }
 
-    /**
-     * Returns the account codes used in the form.
-     *
-     * @return {string[]} the account codes used in the form
-     */
-    getAccountCodesUsed() {
-        return this.#entries.filter((entry) => entry.getAccountCode() !== null).map((entry) => entry.getAccountCode());
-    }
-
     /**
      * Validates the form.
      *
@@ -624,7 +714,7 @@ class DebitCreditSideSubForm {
     validate() {
         let isValid = true;
         isValid = this.#validateReal() && isValid;
-        for (const entry of this.#entries) {
+        for (const entry of this.entries) {
             isValid = entry.validate() && isValid;
         }
         return isValid;
@@ -636,7 +726,7 @@ class DebitCreditSideSubForm {
      * @returns {boolean} true if valid, or false otherwise
      */
     #validateReal() {
-        if (this.#entries.length === 0) {
+        if (this.entries.length === 0) {
             this.#element.classList.add("is-invalid");
             this.#error.innerText = A_("Please add some journal entries.");
             return false;
@@ -677,6 +767,12 @@ class JournalEntrySubForm {
      */
     entryIndex;
 
+    /**
+     * Whether this is an original entry with offsets
+     * @type {boolean}
+     */
+    isMatched;
+
     /**
      * The prefix of the HTML ID and class
      * @type {string}
@@ -725,11 +821,29 @@ class JournalEntrySubForm {
      */
     #summaryText;
 
+    /**
+     * The ID of the original entry
+     * @type {HTMLInputElement}
+     */
+    #originalEntryId;
+
+    /**
+     * The text of the original entry
+     * @type {HTMLDivElement}
+     */
+    #originalEntryText;
+
+    /**
+     * The offset entries
+     * @type {HTMLInputElement}
+     */
+    #offsets;
+
     /**
      * The amount
      * @type {HTMLInputElement}
      */
-    amount;
+    #amount;
 
     /**
      * The text display of the amount
@@ -754,6 +868,7 @@ class JournalEntrySubForm {
         this.element = element;
         this.entryType = element.dataset.entryType;
         this.entryIndex = parseInt(element.dataset.entryIndex);
+        this.isMatched = element.classList.contains("accounting-matched-entry");
         this.#prefix = "accounting-currency-" + element.dataset.currencyIndex + "-" + this.entryType + "-" + this.entryIndex;
         this.#control = document.getElementById(this.#prefix + "-control");
         this.#error = document.getElementById(this.#prefix + "-error");
@@ -762,11 +877,14 @@ class JournalEntrySubForm {
         this.#accountText = document.getElementById(this.#prefix + "-account-text");
         this.#summary = document.getElementById(this.#prefix + "-summary");
         this.#summaryText = document.getElementById(this.#prefix + "-summary-text");
-        this.amount = document.getElementById(this.#prefix + "-amount");
+        this.#originalEntryId = document.getElementById(this.#prefix + "-original-entry-id");
+        this.#originalEntryText = document.getElementById(this.#prefix + "-original-entry-text");
+        this.#offsets = document.getElementById(this.#prefix + "-offsets");
+        this.#amount = document.getElementById(this.#prefix + "-amount");
         this.#amountText = document.getElementById(this.#prefix + "-amount-text");
         this.deleteButton = document.getElementById(this.#prefix + "-delete");
         this.#control.onclick = () => {
-            JournalEntryEditor.edit(this, this.#summary.value, this.#accountCode.value, this.#accountCode.dataset.text, this.amount.value);
+            JournalEntryEditor.edit(this, this.#originalEntryId.value, this.#originalEntryId.dataset.date, this.#originalEntryId.dataset.text, this.#summary.value, this.#accountCode.value, this.#accountCode.dataset.text, this.#amount.value, this.#amount.dataset.min);
         };
         this.deleteButton.onclick = () => {
             this.element.parentElement.removeChild(this.element);
@@ -774,6 +892,33 @@ class JournalEntrySubForm {
         };
     }
 
+    /**
+     * Returns whether the entry is an original entry.
+     *
+     * @return {boolean} true if the entry is an original entry, or false otherwise
+     */
+    isOriginalEntry() {
+        return "isOriginalEntry" in this.element.dataset;
+    }
+
+    /**
+     * Returns the ID of the original entry.
+     *
+     * @return {string|null} the ID of the original entry
+     */
+    getOriginalEntryId() {
+        return this.#originalEntryId.value === ""? null: this.#originalEntryId.value;
+    }
+
+    /**
+     * Returns the date of the original entry.
+     *
+     * @return {string|null} the date of the original entry
+     */
+    getOriginalEntryDate() {
+        return this.#originalEntryId.dataset.date === ""? null: this.#originalEntryId.dataset.date;
+    }
+
     /**
      * Returns the account code.
      *
@@ -783,6 +928,15 @@ class JournalEntrySubForm {
         return this.#accountCode.value === ""? null: this.#accountCode.value;
     }
 
+    /**
+     * Returns the amount.
+     *
+     * @return {Decimal|null} the amount
+     */
+    getAmount() {
+        return this.#amount.value === ""? null: new Decimal(this.#amount.value);
+    }
+
     /**
      * Validates the form.
      *
@@ -794,7 +948,7 @@ class JournalEntrySubForm {
             this.#error.innerText = A_("Please select the account.");
             return false;
         }
-        if (this.amount.value === "") {
+        if (this.#amount.value === "") {
             this.#control.classList.add("is-invalid");
             this.#error.innerText = A_("Please fill in the amount.");
             return false;
@@ -807,21 +961,42 @@ class JournalEntrySubForm {
     /**
      * Stores the data into the journal entry sub-form.
      *
+     * @param isOriginalEntry {boolean} true if this is an original entry, or false otherwise
+     * @param originalEntryId {string} the ID of the original entry
+     * @param originalEntryDate {string} the date of the original entry
+     * @param originalEntryText {string} the text of the original entry
      * @param accountCode {string} the account code
      * @param accountText {string} the account text
      * @param summary {string} the summary
      * @param amount {string} the amount
      */
-    save(accountCode, accountText, summary, amount) {
+    save(isOriginalEntry, originalEntryId, originalEntryDate, originalEntryText, accountCode, accountText, summary, amount) {
+        if (isOriginalEntry) {
+            this.#offsets.classList.remove("d-none");
+        } else {
+            this.#offsets.classList.add("d-none");
+        }
+        this.#originalEntryId.value = originalEntryId;
+        this.#originalEntryId.dataset.date = originalEntryDate;
+        this.#originalEntryId.dataset.text = originalEntryText;
+        if (originalEntryText === "") {
+            this.#originalEntryText.classList.add("d-none");
+            this.#originalEntryText.innerText = "";
+        } else {
+            this.#originalEntryText.classList.remove("d-none");
+            this.#originalEntryText.innerText = A_("Offset %(entry)s", {entry: originalEntryText});
+        }
         this.#accountCode.value = accountCode;
         this.#accountCode.dataset.text = accountText;
         this.#accountText.innerText = accountText;
         this.#summary.value = summary;
         this.#summaryText.innerText = summary;
-        this.amount.value = amount;
+        this.#amount.value = amount;
         this.#amountText.innerText = formatDecimal(new Decimal(amount));
         this.validate();
         this.side.updateTotal();
+        this.side.currency.updateCodeSelectorStatus();
+        this.side.currency.form.updateMinDate();
     }
 }
 

src/accounting/templates/accounting/transaction/expense/detail.html

@@ -35,19 +35,9 @@ First written: 2023/2/26
 
       <ul class="list-group accounting-list-group-stripped accounting-list-group-hover">
         <li class="list-group-item accounting-transaction-entry accounting-transaction-entry-header">{{ A_("Content") }}</li>
-        {% for entry in currency.debit %}
+        {% with entries = currency.debit %}
-          <li class="list-group-item accounting-transaction-entry">
+          {% include "accounting/transaction/include/detail-entries.html" %}
-            <div class="d-flex justify-content-between">
+        {% endwith %}
-              <div>
-                <div class="small">{{ entry.account }}</div>
-                {% if entry.summary is not none %}
-                  <div>{{ entry.summary }}</div>
-                {% endif %}
-              </div>
-              <div>{{ entry.amount|accounting_format_amount }}</div>
-            </div>
-          </li>
-        {% endfor %}
         <li class="list-group-item accounting-transaction-entry accounting-transaction-entry-total">
           <div class="d-flex justify-content-between">
             <div>{{ A_("Total") }}</div>

src/accounting/templates/accounting/transaction/expense/include/form-currency-item.html

@@ -21,15 +21,16 @@ First written: 2023/2/25
 #}
 <div id="accounting-currency-{{ currency_index }}" class="mb-3 accounting-currency" data-index="{{ currency_index }}">
   <input id="accounting-currency-{{ currency_index }}-no" type="hidden" name="currency-{{ currency_index }}-no" value="{{ currency_index }}">
+  <input id="accounting-currency-{{ currency_index }}-code" type="hidden" name="currency-{{ currency_index }}-code" value="{{ currency_code_data }}">
   <div id="accounting-currency-{{ currency_index }}-control" class="form-control accounting-currency-control {% if currency_errors %} is-invalid {% endif %}">
     <div class="d-flex justify-content-between mt-2 mb-3">
       <div class="form-floating accounting-currency-content">
-        <select id="accounting-currency-{{ currency_index }}-code" class="form-select {% if currency_code_errors %} is-invalid {% endif %}" name="currency-{{ currency_index }}-code">
+        <select id="accounting-currency-{{ currency_index }}-code-select" class="form-select {% if currency_code_errors %} is-invalid {% endif %}" name="currency-{{ currency_index }}-code" {% if currency_code_is_locked %} disabled="disabled" {% endif %}>
           {% for currency in accounting_currency_options() %}
             <option value="{{ currency.code }}" {% if currency.code == currency_code_data %} selected="selected" {% endif %}>{{ currency }}</option>
           {% endfor %}
         </select>
-        <label class="form-label" for="accounting-currency-{{ currency_index }}-code">{{ A_("Currency") }}</label>
+        <label class="form-label" for="accounting-currency-{{ currency_index }}-code-select">{{ A_("Currency") }}</label>
         <div id="accounting-currency-{{ currency_index }}-code-error" class="invalid-feedback">{% if currency_code_errors %}{{ currency_code_errors[0] }}{% endif %}</div>
       </div>
 
@@ -55,9 +56,17 @@ First written: 2023/2/25
                     account_text = entry_form.account_text,
                     summary_data = entry_form.summary.data|accounting_default,
                     summary_errors = entry_form.summary.errors,
+                    original_entry_id_data = entry_form.original_entry_id.data|accounting_default,
+                    original_entry_date = entry_form.original_entry_date|accounting_default,
+                    original_entry_text = entry_form.original_entry_text|accounting_default,
+                    is_original_entry = entry_form.is_original_entry,
+                    offset_entries = entry_form.offsets,
+                    offset_total = entry_form.offset_total|accounting_default("0"),
+                    net_balance_data = entry_form.net_balance,
+                    net_balance_text = entry_form.net_balance|accounting_format_amount,
                     amount_data = entry_form.amount.data|accounting_txn_format_amount_input,
                     amount_errors = entry_form.amount.errors,
-                    amount_text = entry_form.amount.data|accounting_format_amount|accounting_default("-"),
+                    amount_text = entry_form.amount.data|accounting_format_amount,
                     entry_errors = entry_form.all_errors %}
               {% include "accounting/transaction/include/form-entry-item.html" %}
             {% endwith %}

src/accounting/templates/accounting/transaction/expense/include/form.html

@@ -29,6 +29,7 @@ First written: 2023/2/25
               currency_errors = currency_form.whole_form.errors,
               currency_code_data = currency_form.code.data,
               currency_code_errors = currency_form.code.errors,
+              currency_code_is_locked = currency_form.is_code_locked,
               debit_forms = currency_form.debit,
               debit_errors = currency_form.debit_errors,
               debit_total = currency_form.form.debit_total|accounting_format_amount %}

src/accounting/templates/accounting/transaction/include/account-selector-modal.html

@@ -37,7 +37,7 @@ First written: 2023/2/25
 
         <ul id="accounting-account-selector-{{ entry_type }}-option-list" class="list-group accounting-selector-list">
           {% for account in account_options %}
-            <li id="accounting-account-selector-{{ entry_type }}-option-{{ account.code }}" class="list-group-item accounting-clickable accounting-account-selector-{{ entry_type }}-option {% if account.is_in_use %} accounting-account-in-use {% endif %}" data-code="{{ account.code }}" data-content="{{ account }}" data-query-values="{{ account.query_values|tojson|forceescape }}" data-bs-toggle="modal" data-bs-target="#accounting-entry-editor-modal">
+            <li id="accounting-account-selector-{{ entry_type }}-option-{{ account.code }}" class="list-group-item accounting-clickable accounting-account-selector-{{ entry_type }}-option {% if account.is_in_use %} accounting-account-in-use {% endif %} {% if account.is_offset_needed %} accounting-account-is-offset-needed {% endif %}" data-code="{{ account.code }}" data-content="{{ account }}" data-query-values="{{ account.query_values|tojson|forceescape }}" data-bs-toggle="modal" data-bs-target="#accounting-entry-editor-modal">
               {{ account }}
             </li>
           {% endfor %}

src/accounting/templates/accounting/transaction/include/detail-entries.html

@@ -0,0 +1,75 @@
+{#
+The Mia! Accounting Flask Project
+detail-entries-item: The journal entries in the transaction detail
+
+ Copyright (c) 2023 imacat.
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+Author: imacat@mail.imacat.idv.tw (imacat)
+First written: 2023/3/14
+#}
+{# <ul> For SonarQube not to complain about incorrect HTML #}
+{% for entry in entries %}
+  <li class="list-group-item accounting-transaction-entry">
+    <div class="d-flex justify-content-between">
+      <div>
+        <div class="small">{{ entry.account }}</div>
+        {% if entry.summary is not none %}
+          <div>{{ entry.summary }}</div>
+        {% endif %}
+        {% if entry.original_entry %}
+          <div class="fst-italic small accounting-original-entry">
+            <a href="{{ url_for("accounting.transaction.detail", txn=entry.original_entry.transaction)|accounting_append_next }}">
+              {{ A_("Offset %(entry)s", entry=entry.original_entry) }}
+            </a>
+          </div>
+        {% endif %}
+        {% if entry.is_original_entry %}
+          <div class="fst-italic small accounting-offset-entries">
+            {% if entry.offsets %}
+              <div class="d-flex justify-content-between">
+                <div>{{ A_("Offsets") }}</div>
+                <ul class="ms-2 ps-0">
+                  {% for offset in entry.offsets %}
+                    <li>
+                      <a href="{{ url_for("accounting.transaction.detail", txn=offset.transaction)|accounting_append_next }}">
+                        {{ offset.transaction.date|accounting_format_date }} {{ offset.amount|accounting_format_amount }}
+                      </a>
+                    </li>
+                  {% endfor %}
+                </ul>
+              </div>
+              {% if entry.balance %}
+                <div class="d-flex justify-content-between">
+                  <div>{{ A_("Net balance") }}</div>
+                  <div>{{ entry.balance|accounting_format_amount }}</div>
+                </div>
+              {% else %}
+                <div class="d-flex justify-content-between">
+                  <div>{{ A_("Fully offset") }}</div>
+                </div>
+              {% endif %}
+            {% else %}
+              <div class="d-flex justify-content-between">
+                {{ A_("Unmatched") }}
+              </div>
+            {% endif %}
+          </div>
+        {% endif %}
+      </div>
+      <div>{{ entry.amount|accounting_format_amount }}</div>
+    </div>
+  </li>
+{% endfor %}
+{# </ul> For SonarQube not to complain about incorrect HTML #}

src/accounting/templates/accounting/transaction/include/detail.html

@@ -42,10 +42,17 @@ First written: 2023/2/26
   </a>
   {% if accounting_can_edit() %}
     {% block to_transfer %}{% endblock %}
+    {% if obj.can_delete %}
       <button class="btn btn-danger" type="button" data-bs-toggle="modal" data-bs-target="#accounting-delete-modal">
         <i class="fa-solid fa-trash"></i>
         {{ A_("Delete") }}
       </button>
+    {% else %}
+      <button class="btn btn-secondary" type="button" disabled="disabled">
+        <i class="fa-solid fa-trash"></i>
+        {{ A_("Delete") }}
+      </button>
+    {% endif %}
   {% endif %}
 </div>
 
@@ -57,7 +64,7 @@ First written: 2023/2/26
   </div>
 {% endif %}
 
-{% if accounting_can_edit() %}
+{% if accounting_can_edit() and obj.can_delete %}
   <form action="{{ url_for("accounting.transaction.delete", txn=obj) }}" method="post">
     <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
     {% if request.args.next %}

src/accounting/templates/accounting/transaction/include/form-entry-item.html

@@ -20,19 +20,45 @@ Author: imacat@mail.imacat.idv.tw (imacat)
 First written: 2023/2/25
 #}
 {# <ul> For SonarQube not to complain about incorrect HTML #}
-<li id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}" class="list-group-item list-group-item-action d-flex justify-content-between accounting-entry accounting-currency-{{ currency_index }}-{{ entry_type }}" data-currency-index="{{ currency_index }}" data-entry-type="{{ entry_type }}" data-entry-index="{{ entry_index }}">
+<li id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}" class="list-group-item list-group-item-action d-flex justify-content-between accounting-currency-{{ currency_index }}-{{ entry_type }} {% if offset_entries %} accounting-matched-entry {% endif %}" data-currency-index="{{ currency_index }}" data-entry-type="{{ entry_type }}" data-entry-index="{{ entry_index }}" {% if is_original_entry %} data-is-original-entry="true" {% endif %}>
   {% if entry_id %}
     <input type="hidden" name="currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-eid" value="{{ entry_id }}">
   {% endif %}
   <input id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-no" type="hidden" name="currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-no" value="{{ entry_index }}">
+  <input id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-original-entry-id" class="accounting-original-entry-id" type="hidden" name="currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-original_entry_id" value="{{ original_entry_id_data }}" data-date="{{ original_entry_date }}" data-text="{{ original_entry_text }}">
   <input id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-account-code" type="hidden" name="currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-account_code" value="{{ account_code_data }}" data-text="{{ account_text }}">
   <input id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-summary" type="hidden" name="currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-summary" value="{{ summary_data }}">
-  <input id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-amount" type="hidden" name="currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-amount" value="{{ amount_data }}">
+  <input id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-amount" type="hidden" name="currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-amount" value="{{ amount_data }}" data-min="{{ offset_total }}">
   <div class="accounting-entry-content">
-    <div id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-control" class="form-control clickable d-flex justify-content-between accounting-entry-control {% if entry_errors %} is-invalid {% endif %}" data-bs-toggle="modal" data-bs-target="#accounting-entry-editor-modal">
+    <div id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-control" class="form-control clickable d-flex justify-content-between {% if entry_errors %} is-invalid {% endif %}" data-bs-toggle="modal" data-bs-target="#accounting-entry-editor-modal">
       <div>
         <div id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-account-text" class="small">{{ account_text }}</div>
         <div id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-summary-text">{{ summary_data }}</div>
+        <div id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-original-entry-text" class="fst-italic small accounting-original-entry {% if not original_entry_text %} d-none {% endif %}">
+          {% if original_entry_text %}{{ A_("Offset %(entry)s", entry=original_entry_text) }}{% endif %}
+        </div>
+        <div id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-offsets" class="fst-italic small accounting-offset-entries {% if not is_original_entry %} d-none {% endif %}">
+          {% if offset_entries %}
+            <div class="d-flex justify-content-between {% if not offset_entries %} d-none {% endif %}">
+              <div>{{ A_("Offsets") }}</div>
+              <ul class="ms-2 ps-0">
+                {% for offset in offset_entries %}
+                  <li>{{ offset.transaction.date|accounting_format_date }} {{ offset.amount|accounting_format_amount }}</li>
+                {% endfor %}
+              </ul>
+            </div>
+            {% if net_balance_data == 0 %}
+              <div>{{ A_("Fully offset") }}</div>
+            {% else %}
+              <div class="d-flex justify-content-between">
+                <div>{{ A_("Net balance") }}</div>
+                <div>{{ net_balance_text }}</div>
+              </div>
+            {% endif %}
+          {% else %}
+            {{ A_("Unmatched") }}
+          {% endif %}
+        </div>
       </div>
       <div><span id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-amount-text" class="badge rounded-pill bg-primary">{{ amount_text }}</span></div>
     </div>
@@ -40,7 +66,7 @@ First written: 2023/2/25
   </div>
 
   <div>
-    <button id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-delete" class="btn btn-danger rounded-circle {% if only_one_entry_form %} d-none {% endif %}" type="button" data-target="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}" data-same-class="accounting-currency-{{ currency_index }}-{{ entry_type }}-btn-delete-entry">
+    <button id="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}-delete" class="btn btn-danger rounded-circle {% if only_one_entry_form or offset_entries %} d-none {% endif %}" type="button" data-target="accounting-currency-{{ currency_index }}-{{ entry_type }}-{{ entry_index }}">
       <i class="fas fa-minus"></i>
     </button>
   </div>

src/accounting/templates/accounting/transaction/include/form.html

@@ -26,6 +26,7 @@ First written: 2023/2/26
   <script src="{{ url_for("accounting.static", filename="js/transaction-form.js") }}"></script>
   <script src="{{ url_for("accounting.static", filename="js/journal-entry-editor.js") }}"></script>
   <script src="{{ url_for("accounting.static", filename="js/account-selector.js") }}"></script>
+  <script src="{{ url_for("accounting.static", filename="js/original-entry-selector.js") }}"></script>
   <script src="{{ url_for("accounting.static", filename="js/summary-editor.js") }}"></script>
 {% endblock %}
 
@@ -45,7 +46,7 @@ First written: 2023/2/26
   {% endif %}
 
   <div class="form-floating mb-3">
-    <input id="accounting-date" class="form-control {% if form.date.errors %} is-invalid {% endif %}" type="date" name="date" value="{{ form.date.data|accounting_default }}" placeholder=" " required="required">
+    <input id="accounting-date" class="form-control {% if form.date.errors %} is-invalid {% endif %}" type="date" name="date" value="{{ form.date.data|accounting_default }}" max="{{ form.max_date|accounting_default }}" min="{{ form.min_date|accounting_default }}" placeholder=" " required="required">
     <label class="form-label" for="accounting-date">{{ A_("Date") }}</label>
     <div id="accounting-date-error" class="invalid-feedback">{% if form.date.errors %}{{ form.date.errors[0] }}{% endif %}</div>
   </div>
@@ -89,5 +90,6 @@ First written: 2023/2/26
 
 {% include "accounting/transaction/include/journal-entry-editor-modal.html" %}
 {% block form_modals %}{% endblock %}
+{% include "accounting/transaction/include/original-entry-selector-modal.html" %}
 
 {% endblock %}

src/accounting/templates/accounting/transaction/include/journal-entry-editor-modal.html

@@ -28,6 +28,22 @@ First written: 2023/2/25
           <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="{{ A_("Close") }}"></button>
         </div>
         <div class="modal-body">
+          <div id="accounting-entry-editor-original-entry-container" class="d-flex justify-content-between mb-3">
+            <div class="accounting-entry-editor-original-entry-content">
+              <div id="accounting-entry-editor-original-entry-control" class="form-control accounting-clickable accounting-material-text-field" data-bs-toggle="modal" data-bs-target="#accounting-original-entry-selector-modal">
+                <label class="form-label" for="accounting-entry-editor-original-entry">{{ A_("Original Entry") }}</label>
+                <div id="accounting-entry-editor-original-entry" data-id="" data-date="" data-text=""></div>
+              </div>
+              <div id="accounting-entry-editor-original-entry-error" class="invalid-feedback"></div>
+            </div>
+
+            <div>
+              <button id="accounting-entry-editor-original-entry-delete" class="btn btn-danger rounded-circle" type="button">
+                <i class="fas fa-minus"></i>
+              </button>
+            </div>
+          </div>
+
           <div class="mb-3">
             <div id="accounting-entry-editor-summary-control" class="form-control accounting-clickable accounting-material-text-field" data-bs-toggle="modal" data-bs-target="">
               <label class="form-label" for="accounting-entry-editor-summary">{{ A_("Summary") }}</label>
@@ -45,7 +61,7 @@ First written: 2023/2/25
           </div>
 
           <div class="form-floating mb-3">
-            <input id="accounting-entry-editor-amount" class="form-control" type="number" value="" min="0.01" max="" step="0.01" placeholder=" " required="required">
+            <input id="accounting-entry-editor-amount" class="form-control" type="number" value="" min="0" max="" step="0.01" placeholder=" " required="required">
             <label for="accounting-entry-editor-amount">{{ A_("Amount") }}</label>
             <div id="accounting-entry-editor-amount-error" class="invalid-feedback"></div>
           </div>

src/accounting/templates/accounting/transaction/include/original-entry-selector-modal.html

@@ -0,0 +1,56 @@
+{#
+The Mia! Accounting Flask Project
+original-entry-selector-modal.html: The modal of the original entry selector
+
+ Copyright (c) 2023 imacat.
+
+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+Author: imacat@mail.imacat.idv.tw (imacat)
+First written: 2023/2/25
+#}
+<div id="accounting-original-entry-selector-modal" class="modal fade" data-currency-code="" data-entry-type="" tabindex="-1" aria-labelledby="accounting-original-entry-selector-modal-label" aria-hidden="true">
+  <div class="modal-dialog">
+    <div class="modal-content">
+      <div class="modal-header">
+        <h1 class="modal-title fs-5" id="accounting-original-entry-selector-modal-label">{{ A_("Select Original Entry") }}</h1>
+        <button type="button" class="btn-close" data-bs-toggle="modal" data-bs-target="#accounting-entry-editor-modal" aria-label="{{ A_("Close") }}"></button>
+      </div>
+      <div class="modal-body">
+        <div class="input-group mb-2">
+          <input id="accounting-original-entry-selector-query" class="form-control form-control-sm" type="search" placeholder=" " required="required">
+          <label class="input-group-text" for="accounting-original-entry-selector-query">
+            <i class="fa-solid fa-magnifying-glass"></i>
+            {{ A_("Search") }}
+          </label>
+        </div>
+
+        <ul id="accounting-original-entry-selector-option-list" class="list-group accounting-selector-list">
+          {% for entry in form.original_entry_options %}
+            <li id="accounting-original-entry-selector-option-{{ entry.id }}" class="list-group-item d-flex justify-content-between accounting-clickable accounting-original-entry-selector-option" data-id="{{ entry.id }}" data-date="{{ entry.transaction.date }}" data-entry-type="{{ "debit" if entry.is_debit else "credit" }}" data-currency-code="{{ entry.currency.code }}" data-account-code="{{ entry.account_code }}" data-account-text="{{ entry.account }}" data-summary="{{ entry.summary|accounting_default }}" data-net-balance="{{ entry.net_balance|accounting_txn_format_amount_input }}" data-text="{{ entry }}" data-query-values="{{ entry.query_values|tojson|forceescape }}" data-bs-toggle="modal" data-bs-target="#accounting-entry-editor-modal">
+              <div>{{ entry.transaction.date|accounting_format_date }} {{ entry.summary|accounting_default }}</div>
+              <div>
+                <span class="badge bg-primary rounded-pill">
+                  <span id="accounting-original-entry-selector-option-{{ entry.id }}-net-balance">{{ entry.net_balance|accounting_format_amount }}</span>
+                  / {{ entry.amount|accounting_format_amount }}
+                </span>
+              </div>
+            </li>
+          {% endfor %}
+        </ul>
+        <p id="accounting-original-entry-selector-option-no-result" class="d-none">{{ A_("There is no data.") }}</p>
+      </div>
+    </div>
+  </div>
+</div>
+

src/accounting/templates/accounting/transaction/include/summary-editor-modal.html

@@ -30,8 +30,11 @@ First written: 2023/2/28
           <button class="btn-close" type="button" data-bs-toggle="modal" data-bs-target="#accounting-entry-editor-modal" aria-label="{{ A_("Close") }}"></button>
         </div>
         <div class="modal-body">
-          <div class="mb-3">
+          <div class="d-flex justify-content-between mb-3">
             <input id="accounting-summary-editor-{{ summary_editor.type }}-summary" class="form-control" type="text" aria-labelledby="accounting-summary-editor-{{ summary_editor.type }}-modal-label">
+            <button id="accounting-summary-editor-{{ summary_editor.type }}-offset" class="btn btn-primary ms-2" type="button" data-bs-toggle="modal" data-bs-target="#accounting-original-entry-selector-modal">
+              {{ A_("Offset...") }}
+            </button>
           </div>
 
           {# Tab navigation #}
@@ -174,7 +177,7 @@ First written: 2023/2/28
           {# The suggested accounts #}
           <div class="mt-3">
             {% for account in summary_editor.accounts %}
-              <button class="btn btn-outline-primary d-none accounting-summary-editor-{{ summary_editor.type }}-account" type="button" data-code="{{ account.code }}" data-text="{{ account }}">
+              <button class="btn btn-outline-primary d-none accounting-summary-editor-{{ summary_editor.type }}-account {% if account.is_offset_needed %} accounting-account-is-offset-needed {% endif %}" type="button" data-code="{{ account.code }}" data-text="{{ account }}">
                 {{ account }}
               </button>
             {% endfor %}

src/accounting/templates/accounting/transaction/income/detail.html

@@ -35,19 +35,9 @@ First written: 2023/2/26
 
       <ul class="list-group accounting-list-group-stripped accounting-list-group-hover">
         <li class="list-group-item accounting-transaction-entry accounting-transaction-entry-header">{{ A_("Content") }}</li>
-        {% for entry in currency.credit %}
+        {% with entries = currency.credit %}
-          <li class="list-group-item accounting-transaction-entry">
+          {% include "accounting/transaction/include/detail-entries.html" %}
-            <div class="d-flex justify-content-between">
+        {% endwith %}
-              <div>
-                <div class="small">{{ entry.account }}</div>
-                {% if entry.summary is not none %}
-                  <div>{{ entry.summary }}</div>
-                {% endif %}
-              </div>
-              <div>{{ entry.amount|accounting_format_amount }}</div>
-            </div>
-          </li>
-        {% endfor %}
         <li class="list-group-item accounting-transaction-entry accounting-transaction-entry-total">
           <div class="d-flex justify-content-between">
             <div>{{ A_("Total") }}</div>

src/accounting/templates/accounting/transaction/income/include/form-currency-item.html

@@ -21,15 +21,16 @@ First written: 2023/2/25
 #}
 <div id="accounting-currency-{{ currency_index }}" class="mb-3 accounting-currency" data-index="{{ currency_index }}">
   <input id="accounting-currency-{{ currency_index }}-no" type="hidden" name="currency-{{ currency_index }}-no" value="{{ currency_index }}">
+  <input id="accounting-currency-{{ currency_index }}-code" type="hidden" name="currency-{{ currency_index }}-code" value="{{ currency_code_data }}">
   <div id="accounting-currency-{{ currency_index }}-control" class="form-control accounting-currency-control {% if currency_errors %} is-invalid {% endif %}">
     <div class="d-flex justify-content-between mt-2 mb-3">
       <div class="form-floating accounting-currency-content">
-        <select id="accounting-currency-{{ currency_index }}-code" class="form-select {% if currency_code_errors %} is-invalid {% endif %}" name="currency-{{ currency_index }}-code">
+        <select id="accounting-currency-{{ currency_index }}-code-select" class="form-select {% if currency_code_errors %} is-invalid {% endif %}" name="currency-{{ currency_index }}-code" {% if currency_code_is_locked %} disabled="disabled" {% endif %}>
           {% for currency in accounting_currency_options() %}
             <option value="{{ currency.code }}" {% if currency.code == currency_code_data %} selected="selected" {% endif %}>{{ currency }}</option>
           {% endfor %}
         </select>
-        <label class="form-label" for="accounting-currency-{{ currency_index }}-code">{{ A_("Currency") }}</label>
+        <label class="form-label" for="accounting-currency-{{ currency_index }}-code-select">{{ A_("Currency") }}</label>
         <div id="accounting-currency-{{ currency_index }}-code-error" class="invalid-feedback">{% if currency_code_errors %}{{ currency_code_errors[0] }}{% endif %}</div>
       </div>
 
@@ -55,9 +56,17 @@ First written: 2023/2/25
                     account_text = entry_form.account_text,
                     summary_data = entry_form.summary.data|accounting_default,
                     summary_errors = entry_form.summary.errors,
+                    original_entry_id_data = entry_form.original_entry_id.data|accounting_default,
+                    original_entry_date = entry_form.original_entry_date|accounting_default,
+                    original_entry_text = entry_form.original_entry_text|accounting_default,
+                    is_original_entry = entry_form.is_original_entry,
+                    offset_entries = entry_form.offsets,
+                    offset_total = entry_form.offset_total|accounting_default("0"),
+                    net_balance_data = entry_form.net_balance,
+                    net_balance_text = entry_form.net_balance|accounting_format_amount,
                     amount_data = entry_form.amount.data|accounting_txn_format_amount_input,
                     amount_errors = entry_form.amount.errors,
-                    amount_text = entry_form.amount.data|accounting_format_amount|accounting_default("-"),
+                    amount_text = entry_form.amount.data|accounting_format_amount,
                     entry_errors = entry_form.all_errors %}
               {% include "accounting/transaction/include/form-entry-item.html" %}
             {% endwith %}

src/accounting/templates/accounting/transaction/income/include/form.html

@@ -29,6 +29,7 @@ First written: 2023/2/25
               currency_errors = currency_form.whole_form.errors,
               currency_code_data = currency_form.code.data,
               currency_code_errors = currency_form.code.errors,
+              currency_code_is_locked = currency_form.is_code_locked,
               credit_forms = currency_form.credit,
               credit_errors = currency_form.credit_errors,
               credit_total = currency_form.form.credit_total|accounting_format_amount %}

src/accounting/templates/accounting/transaction/transfer/detail.html

@@ -31,19 +31,9 @@ First written: 2023/2/26
         <div class="col-sm-6 mb-2">
           <ul class="list-group accounting-list-group-stripped accounting-list-group-hover">
             <li class="list-group-item accounting-transaction-entry accounting-transaction-entry-header">{{ A_("Debit") }}</li>
-            {% for entry in currency.debit %}
+            {% with entries = currency.debit %}
-              <li class="list-group-item accounting-transaction-entry">
+              {% include "accounting/transaction/include/detail-entries.html" %}
-                <div class="d-flex justify-content-between">
+            {% endwith %}
-                  <div>
-                    <div class="small">{{ entry.account }}</div>
-                    {% if entry.summary is not none %}
-                      <div>{{ entry.summary }}</div>
-                    {% endif %}
-                  </div>
-                  <div>{{ entry.amount|accounting_format_amount }}</div>
-                </div>
-              </li>
-            {% endfor %}
             <li class="list-group-item accounting-transaction-entry accounting-transaction-entry-total">
               <div class="d-flex justify-content-between">
                 <div>{{ A_("Total") }}</div>
@@ -57,19 +47,9 @@ First written: 2023/2/26
         <div class="col-sm-6 mb-2">
           <ul class="list-group accounting-list-group-stripped accounting-list-group-hover">
             <li class="list-group-item accounting-transaction-entry accounting-transaction-entry-header">{{ A_("Credit") }}</li>
-            {% for entry in currency.credit %}
+            {% with entries = currency.credit %}
-              <li class="list-group-item accounting-transaction-entry">
+              {% include "accounting/transaction/include/detail-entries.html" %}
-                <div class="d-flex justify-content-between">
+            {% endwith %}
-                  <div>
-                    <div class="small">{{ entry.account }}</div>
-                    {% if entry.summary is not none %}
-                      <div>{{ entry.summary }}</div>
-                    {% endif %}
-                  </div>
-                  <div>{{ entry.amount|accounting_format_amount }}</div>
-                </div>
-              </li>
-            {% endfor %}
             <li class="list-group-item accounting-transaction-entry accounting-transaction-entry-total">
               <div class="d-flex justify-content-between">
                 <div>{{ A_("Total") }}</div>

src/accounting/templates/accounting/transaction/transfer/include/form-currency-item.html

@@ -21,15 +21,16 @@ First written: 2023/2/25
 #}
 <div id="accounting-currency-{{ currency_index }}" class="mb-3 accounting-currency" data-index="{{ currency_index }}">
   <input id="accounting-currency-{{ currency_index }}-no" type="hidden" name="currency-{{ currency_index }}-no" value="{{ currency_index }}">
+  <input id="accounting-currency-{{ currency_index }}-code" type="hidden" name="currency-{{ currency_index }}-code" value="{{ currency_code_data }}">
   <div id="accounting-currency-{{ currency_index }}-control" class="form-control accounting-currency-control {% if currency_errors %} is-invalid {% endif %}">
     <div class="d-flex justify-content-between mt-2 mb-3">
       <div class="form-floating accounting-currency-content">
-        <select id="accounting-currency-{{ currency_index }}-code" class="form-select {% if currency_code_errors %} is-invalid {% endif %}" name="currency-{{ currency_index }}-code">
+        <select id="accounting-currency-{{ currency_index }}-code-select" class="form-select {% if currency_code_errors %} is-invalid {% endif %}" {% if currency_code_is_locked %} disabled="disabled" {% endif %}>
           {% for currency in accounting_currency_options() %}
             <option value="{{ currency.code }}" {% if currency.code == currency_code_data %} selected="selected" {% endif %}>{{ currency }}</option>
           {% endfor %}
         </select>
-        <label class="form-label" for="accounting-currency-{{ currency_index }}-code">{{ A_("Currency") }}</label>
+        <label class="form-label" for="accounting-currency-{{ currency_index }}-code-select">{{ A_("Currency") }}</label>
         <div id="accounting-currency-{{ currency_index }}-code-error" class="invalid-feedback">{% if currency_code_errors %}{{ currency_code_errors[0] }}{% endif %}</div>
       </div>
 
@@ -57,9 +58,17 @@ First written: 2023/2/25
                       account_text = entry_form.account_text,
                       summary_data = entry_form.summary.data|accounting_default,
                       summary_errors = entry_form.summary.errors,
+                      original_entry_id_data = entry_form.original_entry_id.data|accounting_default,
+                      original_entry_date = entry_form.original_entry_date|accounting_default,
+                      original_entry_text = entry_form.original_entry_text|accounting_default,
+                      is_original_entry = entry_form.is_original_entry,
+                      offset_entries = entry_form.offsets,
+                      offset_total = entry_form.offset_total|accounting_default,
+                      net_balance_data = entry_form.net_balance,
+                      net_balance_text = entry_form.net_balance|accounting_format_amount,
                       amount_data = entry_form.amount.data|accounting_txn_format_amount_input,
                       amount_errors = entry_form.amount.errors,
-                      amount_text = entry_form.amount.data|accounting_format_amount|accounting_default("-"),
+                      amount_text = entry_form.amount.data|accounting_format_amount,
                       entry_errors = entry_form.all_errors %}
                 {% include "accounting/transaction/include/form-entry-item.html" %}
               {% endwith %}
@@ -97,9 +106,17 @@ First written: 2023/2/25
                       account_text = entry_form.account_text,
                       summary_data = entry_form.summary.data|accounting_default,
                       summary_errors = entry_form.summary.errors,
+                      original_entry_id_data = entry_form.original_entry_id.data|accounting_default,
+                      original_entry_date = entry_form.original_entry_date|accounting_default,
+                      original_entry_text = entry_form.original_entry_text|accounting_default,
+                      is_original_entry = entry_form.is_original_entry,
+                      offset_entries = entry_form.offsets,
+                      offset_total = entry_form.offset_total|accounting_default("0"),
+                      net_balance_data = entry_form.net_balance,
+                      net_balance_text = entry_form.net_balance|accounting_format_amount,
                       amount_data = entry_form.amount.data|accounting_txn_format_amount_input,
                       amount_errors = entry_form.amount.errors,
-                      amount_text = entry_form.amount.data|accounting_format_amount|accounting_default("-"),
+                      amount_text = entry_form.amount.data|accounting_format_amount,
                       entry_errors = entry_form.all_errors %}
                 {% include "accounting/transaction/include/form-entry-item.html" %}
               {% endwith %}

src/accounting/templates/accounting/transaction/transfer/include/form.html

@@ -29,6 +29,7 @@ First written: 2023/2/25
               currency_errors = currency_form.whole_form.errors,
               currency_code_data = currency_form.code.data,
               currency_code_errors = currency_form.code.errors,
+              currency_code_is_locked = currency_form.is_code_locked,
               debit_forms = currency_form.debit,
               debit_errors = currency_form.debit_errors,
               debit_total = currency_form.form.debit_total|accounting_format_amount,

src/accounting/transaction/converters.py

@@ -20,10 +20,10 @@
 from datetime import date
 
 from flask import abort
+from sqlalchemy.orm import selectinload
 from werkzeug.routing import BaseConverter
 
-from accounting import db
+from accounting.models import Transaction, JournalEntry
-from accounting.models import Transaction
 from accounting.utils.txn_types import TransactionType
 
 
@@ -37,7 +37,13 @@ class TransactionConverter(BaseConverter):
         :param value: The transaction ID.
         :return: The corresponding transaction.
         """
-        transaction: Transaction | None = db.session.get(Transaction, value)
+        transaction: Transaction | None = Transaction.query\
+            .join(JournalEntry)\
+            .filter(Transaction.id == value)\
+            .options(selectinload(Transaction.entries)
+                     .selectinload(JournalEntry.offsets)
+                     .selectinload(JournalEntry.transaction))\
+            .first()
         if transaction is None:
             abort(404)
         return transaction

src/accounting/transaction/forms/currency.py

@@ -19,6 +19,7 @@
 """
 from decimal import Decimal
 
+import sqlalchemy as sa
 from flask_babel import LazyString
 from flask_wtf import FlaskForm
 from wtforms import StringField, ValidationError, FieldList, IntegerField, \
@@ -27,9 +28,11 @@ from wtforms.validators import DataRequired
 
 from accounting import db
 from accounting.locale import lazy_gettext
-from accounting.models import Currency
+from accounting.models import Currency, JournalEntry
+from accounting.transaction.utils.offset_alias import offset_alias
+from accounting.utils.cast import be
 from accounting.utils.strip_text import strip_text
-from .journal_entry import CreditEntryForm, DebitEntryForm
+from .journal_entry import JournalEntryForm, CreditEntryForm, DebitEntryForm
 
 CURRENCY_REQUIRED: DataRequired = DataRequired(
     lazy_gettext("Please select the currency."))
@@ -47,6 +50,50 @@ class CurrencyExists:
                 "The currency does not exist."))
 
 
+class SameCurrencyAsOriginalEntries:
+    """The validator to check if the currency is the same as the original
+    entries."""
+
+    def __call__(self, form: FlaskForm, field: StringField) -> None:
+        assert isinstance(form, CurrencyForm)
+        if field.data is None:
+            return
+        original_entry_id: set[int] = {x.original_entry_id.data
+                                       for x in form.entries
+                                       if x.original_entry_id.data is not None}
+        if len(original_entry_id) == 0:
+            return
+        original_entry_currency_codes: set[str] = set(db.session.scalars(
+            sa.select(JournalEntry.currency_code)
+            .filter(JournalEntry.id.in_(original_entry_id))).all())
+        for currency_code in original_entry_currency_codes:
+            if field.data != currency_code:
+                raise ValidationError(lazy_gettext(
+                    "The currency must be the same as the original entry."))
+
+
+class KeepCurrencyWhenHavingOffset:
+    """The validator to check if the currency is the same when there is
+    offset."""
+
+    def __call__(self, form: FlaskForm, field: StringField) -> None:
+        assert isinstance(form, CurrencyForm)
+        if field.data is None:
+            return
+        offset: sa.Alias = offset_alias()
+        original_entries: list[JournalEntry] = JournalEntry.query\
+            .join(offset, be(JournalEntry.id == offset.c.original_entry_id),
+                  isouter=True)\
+            .filter(JournalEntry.id.in_({x.eid.data for x in form.entries
+                                         if x.eid.data is not None}))\
+            .group_by(JournalEntry.id, JournalEntry.currency_code)\
+            .having(sa.func.count(offset.c.id) > 0).all()
+        for original_entry in original_entries:
+            if original_entry.currency_code != field.data:
+                raise ValidationError(lazy_gettext(
+                    "The currency must not be changed when there is offset."))
+
+
 class NeedSomeJournalEntries:
     """The validator to check if there is any journal entry sub-form."""
 
@@ -78,6 +125,41 @@ class CurrencyForm(FlaskForm):
     whole_form = BooleanField()
     """The pseudo field for the whole form validators."""
 
+    @property
+    def entries(self) -> list[JournalEntryForm]:
+        """Returns the journal entry sub-forms.
+
+        :return: The journal entry sub-forms.
+        """
+        entry_forms: list[JournalEntryForm] = []
+        if isinstance(self, IncomeCurrencyForm):
+            entry_forms.extend([x.form for x in self.credit])
+        elif isinstance(self, ExpenseCurrencyForm):
+            entry_forms.extend([x.form for x in self.debit])
+        elif isinstance(self, TransferCurrencyForm):
+            entry_forms.extend([x.form for x in self.debit])
+            entry_forms.extend([x.form for x in self.credit])
+        return entry_forms
+
+    @property
+    def is_code_locked(self) -> bool:
+        """Returns whether the currency code should not be changed.
+
+        :return: True if the currency code should not be changed, or False
+            otherwise
+        """
+        entry_forms: list[JournalEntryForm] = self.entries
+        original_entry_id: set[int] \
+            = {x.original_entry_id.data for x in entry_forms
+               if x.original_entry_id.data is not None}
+        if len(original_entry_id) > 0:
+            return True
+        entry_id: set[int] = {x.eid.data for x in entry_forms
+                              if x.eid.data is not None}
+        select: sa.Select = sa.select(sa.func.count(JournalEntry.id))\
+            .filter(JournalEntry.original_entry_id.in_(entry_id))
+        return db.session.scalar(select) > 0
+
 
 class IncomeCurrencyForm(CurrencyForm):
     """The form to create or edit a currency in a cash income transaction."""
@@ -86,7 +168,9 @@ class IncomeCurrencyForm(CurrencyForm):
     code = StringField(
         filters=[strip_text],
         validators=[CURRENCY_REQUIRED,
-                    CurrencyExists()])
+                    CurrencyExists(),
+                    SameCurrencyAsOriginalEntries(),
+                    KeepCurrencyWhenHavingOffset()])
     """The currency code."""
     credit = FieldList(FormField(CreditEntryForm),
                        validators=[NeedSomeJournalEntries()])
@@ -121,7 +205,9 @@ class ExpenseCurrencyForm(CurrencyForm):
     code = StringField(
         filters=[strip_text],
         validators=[CURRENCY_REQUIRED,
-                    CurrencyExists()])
+                    CurrencyExists(),
+                    SameCurrencyAsOriginalEntries(),
+                    KeepCurrencyWhenHavingOffset()])
     """The currency code."""
     debit = FieldList(FormField(DebitEntryForm),
                       validators=[NeedSomeJournalEntries()])
@@ -156,7 +242,9 @@ class TransferCurrencyForm(CurrencyForm):
     code = StringField(
         filters=[strip_text],
         validators=[CURRENCY_REQUIRED,
-                    CurrencyExists()])
+                    CurrencyExists(),
+                    SameCurrencyAsOriginalEntries(),
+                    KeepCurrencyWhenHavingOffset()])
     """The currency code."""
     debit = FieldList(FormField(DebitEntryForm),
                       validators=[NeedSomeJournalEntries()])

src/accounting/transaction/forms/journal_entry.py

@@ -18,14 +18,21 @@
 
 """
 import re
+from datetime import date
+from decimal import Decimal
 
+import sqlalchemy as sa
 from flask_babel import LazyString
 from flask_wtf import FlaskForm
+from sqlalchemy.orm import selectinload
 from wtforms import StringField, ValidationError, DecimalField, IntegerField
-from wtforms.validators import DataRequired
+from wtforms.validators import DataRequired, Optional
 
+from accounting import db
 from accounting.locale import lazy_gettext
 from accounting.models import Account, JournalEntry
+from accounting.template_filters import format_amount
+from accounting.utils.cast import be
 from accounting.utils.random_id import new_id
 from accounting.utils.strip_text import strip_text
 from accounting.utils.user import get_current_user_pk
@@ -35,6 +42,66 @@ ACCOUNT_REQUIRED: DataRequired = DataRequired(
 """The validator to check if the account code is empty."""
 
 
+class OriginalEntryExists:
+    """The validator to check if the original entry exists."""
+
+    def __call__(self, form: FlaskForm, field: IntegerField) -> None:
+        if field.data is None:
+            return
+        if db.session.get(JournalEntry, field.data) is None:
+            raise ValidationError(lazy_gettext(
+                "The original entry does not exist."))
+
+
+class OriginalEntryOppositeSide:
+    """The validator to check if the original entry is on the opposite side."""
+
+    def __call__(self, form: FlaskForm, field: IntegerField) -> None:
+        if field.data is None:
+            return
+        original_entry: JournalEntry | None \
+            = db.session.get(JournalEntry, field.data)
+        if original_entry is None:
+            return
+        if isinstance(form, CreditEntryForm) and original_entry.is_debit:
+            return
+        if isinstance(form, DebitEntryForm) and not original_entry.is_debit:
+            return
+        raise ValidationError(lazy_gettext(
+            "The original entry is on the same side."))
+
+
+class OriginalEntryNeedOffset:
+    """The validator to check if the original entry needs offset."""
+
+    def __call__(self, form: FlaskForm, field: IntegerField) -> None:
+        if field.data is None:
+            return
+        original_entry: JournalEntry | None \
+            = db.session.get(JournalEntry, field.data)
+        if original_entry is None:
+            return
+        if not original_entry.account.is_offset_needed:
+            raise ValidationError(lazy_gettext(
+                "The original entry does not need offset."))
+
+
+class OriginalEntryNotOffset:
+    """The validator to check if the original entry is not itself an offset
+    entry."""
+
+    def __call__(self, form: FlaskForm, field: IntegerField) -> None:
+        if field.data is None:
+            return
+        original_entry: JournalEntry | None \
+            = db.session.get(JournalEntry, field.data)
+        if original_entry is None:
+            return
+        if original_entry.original_entry_id is not None:
+            raise ValidationError(lazy_gettext(
+                "The original entry cannot be an offset entry."))
+
+
 class AccountExists:
     """The validator to check if the account exists."""
 
@@ -74,6 +141,72 @@ class IsCreditAccount:
             "This account is not for credit entries."))
 
 
+class SameAccountAsOriginalEntry:
+    """The validator to check if the account is the same as the original
+    entry."""
+
+    def __call__(self, form: FlaskForm, field: StringField) -> None:
+        assert isinstance(form, JournalEntryForm)
+        if field.data is None or form.original_entry_id.data is None:
+            return
+        original_entry: JournalEntry | None \
+            = db.session.get(JournalEntry, form.original_entry_id.data)
+        if original_entry is None:
+            return
+        if field.data != original_entry.account_code:
+            raise ValidationError(lazy_gettext(
+                "The account must be the same as the original entry."))
+
+
+class KeepAccountWhenHavingOffset:
+    """The validator to check if the account is the same when having offset."""
+
+    def __call__(self, form: FlaskForm, field: StringField) -> None:
+        assert isinstance(form, JournalEntryForm)
+        if field.data is None or form.eid.data is None:
+            return
+        entry: JournalEntry | None = db.session.query(JournalEntry)\
+            .filter(JournalEntry.id == form.eid.data)\
+            .options(selectinload(JournalEntry.offsets)).first()
+        if entry is None or len(entry.offsets) == 0:
+            return
+        if field.data != entry.account_code:
+            raise ValidationError(lazy_gettext(
+                "The account must not be changed when there is offset."))
+
+
+class NotStartPayableFromDebit:
+    """The validator to check that a payable journal entry does not start from
+    the debit side."""
+
+    def __call__(self, form: FlaskForm, field: StringField) -> None:
+        assert isinstance(form, DebitEntryForm)
+        if field.data is None \
+                or field.data[0] != "2" \
+                or form.original_entry_id.data is not None:
+            return
+        account: Account | None = Account.find_by_code(field.data)
+        if account is not None and account.is_offset_needed:
+            raise ValidationError(lazy_gettext(
+                "A payable entry cannot start from the debit side."))
+
+
+class NotStartReceivableFromCredit:
+    """The validator to check that a receivable journal entry does not start
+    from the credit side."""
+
+    def __call__(self, form: FlaskForm, field: StringField) -> None:
+        assert isinstance(form, CreditEntryForm)
+        if field.data is None \
+                or field.data[0] != "1" \
+                or form.original_entry_id.data is not None:
+            return
+        account: Account | None = Account.find_by_code(field.data)
+        if account is not None and account.is_offset_needed:
+            raise ValidationError(lazy_gettext(
+                "A receivable entry cannot start from the credit side."))
+
+
 class PositiveAmount:
     """The validator to check if the amount is positive."""
 
@@ -85,17 +218,86 @@ class PositiveAmount:
                 "Please fill in a positive amount."))
 
 
+class NotExceedingOriginalEntryNetBalance:
+    """The validator to check if the amount exceeds the net balance of the
+    original entry."""
+
+    def __call__(self, form: FlaskForm, field: DecimalField) -> None:
+        assert isinstance(form, JournalEntryForm)
+        if field.data is None or form.original_entry_id.data is None:
+            return
+        original_entry: JournalEntry | None \
+            = db.session.get(JournalEntry, form.original_entry_id.data)
+        if original_entry is None:
+            return
+        is_debit: bool = isinstance(form, DebitEntryForm)
+        existing_entry_id: set[int] = set()
+        if form.txn_form.obj is not None:
+            existing_entry_id = {x.id for x in form.txn_form.obj.entries}
+        offset_total_func: sa.Function = sa.func.sum(sa.case(
+            (be(JournalEntry.is_debit == is_debit), JournalEntry.amount),
+            else_=-JournalEntry.amount))
+        offset_total_but_form: Decimal | None = db.session.scalar(
+            sa.select(offset_total_func)
+            .filter(be(JournalEntry.original_entry_id == original_entry.id),
+                    JournalEntry.id.not_in(existing_entry_id)))
+        if offset_total_but_form is None:
+            offset_total_but_form = Decimal("0")
+        offset_total_on_form: Decimal = sum(
+            [x.amount.data for x in form.txn_form.entries
+             if x.original_entry_id.data == original_entry.id
+             and x.amount != field and x.amount.data is not None])
+        net_balance: Decimal = original_entry.amount - offset_total_but_form \
+            - offset_total_on_form
+        if field.data > net_balance:
+            raise ValidationError(lazy_gettext(
+                "The amount must not exceed the net balance %(balance)s of the"
+                " original entry.", balance=format_amount(net_balance)))
+
+
+class NotLessThanOffsetTotal:
+    """The validator to check if the amount is less than the offset total."""
+
+    def __call__(self, form: FlaskForm, field: DecimalField) -> None:
+        assert isinstance(form, JournalEntryForm)
+        if field.data is None or form.eid.data is None:
+            return
+        is_debit: bool = isinstance(form, DebitEntryForm)
+        select_offset_total: sa.Select = sa.select(sa.func.sum(sa.case(
+            (JournalEntry.is_debit != is_debit, JournalEntry.amount),
+            else_=-JournalEntry.amount)))\
+            .filter(be(JournalEntry.original_entry_id == form.eid.data))
+        offset_total: Decimal | None = db.session.scalar(select_offset_total)
+        if offset_total is not None and field.data < offset_total:
+            raise ValidationError(lazy_gettext(
+                "The amount must not be less than the offset total %(total)s.",
+                total=format_amount(offset_total)))
+
+
 class JournalEntryForm(FlaskForm):
     """The base form to create or edit a journal entry."""
     eid = IntegerField()
     """The existing journal entry ID."""
     no = IntegerField()
     """The order in the currency."""
+    original_entry_id = IntegerField()
+    """The Id of the original entry."""
     account_code = StringField()
     """The account code."""
     amount = DecimalField()
     """The amount."""
 
+    def __init__(self, *args, **kwargs):
+        """Constructs a base transaction form.
+
+        :param args: The arguments.
+        :param kwargs: The keyword arguments.
+        """
+        super().__init__(*args, **kwargs)
+        from .transaction import TransactionForm
+        self.txn_form: TransactionForm | None = None
+        """The source transaction form."""
+
     @property
     def account_text(self) -> str:
         """Returns the text representation of the account.
@@ -109,6 +311,124 @@ class JournalEntryForm(FlaskForm):
             return ""
         return str(account)
 
+    @property
+    def __original_entry(self) -> JournalEntry | None:
+        """Returns the original entry.
+
+        :return: The original entry.
+        """
+        if not hasattr(self, "____original_entry"):
+            def get_entry() -> JournalEntry | None:
+                if self.original_entry_id.data is None:
+                    return None
+                return db.session.get(JournalEntry,
+                                      self.original_entry_id.data)
+            setattr(self, "____original_entry", get_entry())
+        return getattr(self, "____original_entry")
+
+    @property
+    def original_entry_date(self) -> date | None:
+        """Returns the text representation of the original entry.
+
+        :return: The text representation of the original entry.
+        """
+        return None if self.__original_entry is None \
+            else self.__original_entry.transaction.date
+
+    @property
+    def original_entry_text(self) -> str | None:
+        """Returns the text representation of the original entry.
+
+        :return: The text representation of the original entry.
+        """
+        return None if self.__original_entry is None \
+            else str(self.__original_entry)
+
+    @property
+    def __entry(self) -> JournalEntry | None:
+        """Returns the journal entry.
+
+        :return: The journal entry.
+        """
+        if not hasattr(self, "____entry"):
+            def get_entry() -> JournalEntry | None:
+                if self.eid.data is None:
+                    return None
+                return JournalEntry.query\
+                    .filter(JournalEntry.id == self.eid.data)\
+                    .options(selectinload(JournalEntry.transaction),
+                             selectinload(JournalEntry.account),
+                             selectinload(JournalEntry.offsets)
+                             .selectinload(JournalEntry.transaction))\
+                    .first()
+            setattr(self, "____entry", get_entry())
+        return getattr(self, "____entry")
+
+    @property
+    def is_original_entry(self) -> bool:
+        """Returns whether the entry is an original entry.
+
+        :return: True if the entry is an original entry, or False otherwise.
+        """
+        if self.account_code.data is None:
+            return False
+        if self.account_code.data[0] == "1":
+            if isinstance(self, CreditEntryForm):
+                return False
+        elif self.account_code.data[0] == "2":
+            if isinstance(self, DebitEntryForm):
+                return False
+        else:
+            return False
+        account: Account | None = Account.find_by_code(self.account_code.data)
+        return account is not None and account.is_offset_needed
+
+    @property
+    def offsets(self) -> list[JournalEntry]:
+        """Returns the offsets.
+
+        :return: The offsets.
+        """
+        if not hasattr(self, "__offsets"):
+            def get_offsets() -> list[JournalEntry]:
+                if not self.is_original_entry or self.eid.data is None:
+                    return []
+                return JournalEntry.query\
+                    .filter(JournalEntry.original_entry_id == self.eid.data)\
+                    .options(selectinload(JournalEntry.transaction),
+                             selectinload(JournalEntry.account),
+                             selectinload(JournalEntry.offsets)
+                             .selectinload(JournalEntry.transaction)).all()
+            setattr(self, "__offsets", get_offsets())
+        return getattr(self, "__offsets")
+
+    @property
+    def offset_total(self) -> Decimal | None:
+        """Returns the total amount of the offsets.
+
+        :return: The total amount of the offsets.
+        """
+        if not hasattr(self, "__offset_total"):
+            def get_offset_total():
+                if not self.is_original_entry or self.eid.data is None:
+                    return None
+                is_debit: bool = isinstance(self, DebitEntryForm)
+                return sum([x.amount if x.is_debit != is_debit else -x.amount
+                            for x in self.offsets])
+            setattr(self, "__offset_total", get_offset_total())
+        return getattr(self, "__offset_total")
+
+    @property
+    def net_balance(self) -> Decimal | None:
+        """Returns the net balance.
+
+        :return: The net balance.
+        """
+        if not self.is_original_entry or self.eid.data is None \
+                or self.amount.data is None:
+            return None
+        return self.amount.data - self.offset_total
+
     @property
     def all_errors(self) -> list[str | LazyString]:
         """Returns all the errors of the form.
@@ -128,15 +448,30 @@ class DebitEntryForm(JournalEntryForm):
     """The existing journal entry ID."""
     no = IntegerField()
     """The order in the currency."""
+    original_entry_id = IntegerField(
+        validators=[Optional(),
+                    OriginalEntryExists(),
+                    OriginalEntryOppositeSide(),
+                    OriginalEntryNeedOffset(),
+                    OriginalEntryNotOffset()])
+    """The Id of the original entry."""
     account_code = StringField(
         filters=[strip_text],
         validators=[ACCOUNT_REQUIRED,
                     AccountExists(),
-                    IsDebitAccount()])
+                    IsDebitAccount(),
+                    SameAccountAsOriginalEntry(),
+                    KeepAccountWhenHavingOffset(),
+                    NotStartPayableFromDebit()])
     """The account code."""
+    offset_original_entry_id = IntegerField()
+    """The Id of the original entry."""
     summary = StringField(filters=[strip_text])
     """The summary."""
-    amount = DecimalField(validators=[PositiveAmount()])
+    amount = DecimalField(
+        validators=[PositiveAmount(),
+                    NotExceedingOriginalEntryNetBalance(),
+                    NotLessThanOffsetTotal()])
     """The amount."""
 
     def populate_obj(self, obj: JournalEntry) -> None:
@@ -148,6 +483,7 @@ class DebitEntryForm(JournalEntryForm):
         is_new: bool = obj.id is None
         if is_new:
             obj.id = new_id(JournalEntry)
+        obj.original_entry_id = self.original_entry_id.data
         obj.account_id = Account.find_by_code(self.account_code.data).id
         obj.summary = self.summary.data
         obj.is_debit = True
@@ -164,15 +500,28 @@ class CreditEntryForm(JournalEntryForm):
     """The existing journal entry ID."""
     no = IntegerField()
     """The order in the currency."""
+    original_entry_id = IntegerField(
+        validators=[Optional(),
+                    OriginalEntryExists(),
+                    OriginalEntryOppositeSide(),
+                    OriginalEntryNeedOffset(),
+                    OriginalEntryNotOffset()])
+    """The Id of the original entry."""
     account_code = StringField(
         filters=[strip_text],
         validators=[ACCOUNT_REQUIRED,
                     AccountExists(),
-                    IsCreditAccount()])
+                    IsCreditAccount(),
+                    SameAccountAsOriginalEntry(),
+                    KeepAccountWhenHavingOffset(),
+                    NotStartReceivableFromCredit()])
     """The account code."""
     summary = StringField(filters=[strip_text])
     """The summary."""
-    amount = DecimalField(validators=[PositiveAmount()])
+    amount = DecimalField(
+        validators=[PositiveAmount(),
+                    NotExceedingOriginalEntryNetBalance(),
+                    NotLessThanOffsetTotal()])
     """The amount."""
 
     def populate_obj(self, obj: JournalEntry) -> None:
@@ -184,6 +533,7 @@ class CreditEntryForm(JournalEntryForm):
         is_new: bool = obj.id is None
         if is_new:
             obj.id = new_id(JournalEntry)
+        obj.original_entry_id = self.original_entry_id.data
         obj.account_id = Account.find_by_code(self.account_code.data).id
         obj.summary = self.summary.data
         obj.is_debit = False

src/accounting/transaction/forms/reorder.py

@@ -19,13 +19,14 @@
 """
 from datetime import date
 
+import sqlalchemy as sa
 from flask import request
 
 from accounting import db
 from accounting.models import Transaction
 
 
-def sort_transactions_in(txn_date: date, exclude: int) -> None:
+def sort_transactions_in(txn_date: date, exclude: int | None = None) -> None:
     """Sorts the transactions under a date after changing the date or deleting
     a transaction.
 
@@ -33,9 +34,11 @@ def sort_transactions_in(txn_date: date, exclude: int) -> None:
     :param exclude: The transaction ID to exclude.
     :return: None.
     """
+    conditions: list[sa.BinaryExpression] = [Transaction.date == txn_date]
+    if exclude is not None:
+        conditions.append(Transaction.id != exclude)
     transactions: list[Transaction] = Transaction.query\
-        .filter(Transaction.date == txn_date,
+        .filter(*conditions)\
-                Transaction.id != exclude)\
         .order_by(Transaction.no).all()
     for i in range(len(transactions)):
         if transactions[i].no != i + 1:

src/accounting/transaction/forms/transaction.py

@@ -17,14 +17,15 @@
 """The transaction forms for the transaction management.
 
 """
+import datetime as dt
 import typing as t
 from abc import ABC, abstractmethod
 
 import sqlalchemy as sa
 from flask_babel import LazyString
 from flask_wtf import FlaskForm
-from wtforms import DateField, FieldList, FormField, \
+from wtforms import DateField, FieldList, FormField, TextAreaField, \
-    TextAreaField
+    BooleanField
 from wtforms.validators import DataRequired, ValidationError
 
 from accounting import db
@@ -32,6 +33,8 @@ from accounting.locale import lazy_gettext
 from accounting.models import Transaction, Account, JournalEntry, \
     TransactionCurrency
 from accounting.transaction.utils.account_option import AccountOption
+from accounting.transaction.utils.original_entries import \
+    get_selectable_original_entries
 from accounting.transaction.utils.summary_editor import SummaryEditor
 from accounting.utils.random_id import new_id
 from accounting.utils.strip_text import strip_multiline_text
@@ -46,6 +49,37 @@ DATE_REQUIRED: DataRequired = DataRequired(
 """The validator to check if the date is empty."""
 
 
+class NotBeforeOriginalEntries:
+    """The validator to check if the date is not before the original
+    entries."""
+
+    def __call__(self, form: FlaskForm, field: DateField) -> None:
+        assert isinstance(form, TransactionForm)
+        if field.data is None:
+            return
+        min_date: dt.date | None = form.min_date
+        if min_date is None:
+            return
+        if field.data < min_date:
+            raise ValidationError(lazy_gettext(
+                "The date cannot be earlier than the original entries."))
+
+
+class NotAfterOffsetEntries:
+    """The validator to check if the date is not after the offset entries."""
+
+    def __call__(self, form: FlaskForm, field: DateField) -> None:
+        assert isinstance(form, TransactionForm)
+        if field.data is None:
+            return
+        max_date: dt.date | None = form.max_date
+        if max_date is None:
+            return
+        if field.data > max_date:
+            raise ValidationError(lazy_gettext(
+                "The date cannot be later than the offset entries."))
+
+
 class NeedSomeCurrencies:
     """The validator to check if there is any currency sub-form."""
 
@@ -54,6 +88,23 @@ class NeedSomeCurrencies:
             raise ValidationError(lazy_gettext("Please add some currencies."))
 
 
+class CannotDeleteOriginalEntriesWithOffset:
+    """The validator to check the original entries with offset."""
+
+    def __call__(self, form: FlaskForm, field: FieldList) -> None:
+        assert isinstance(form, TransactionForm)
+        if form.obj is None:
+            return
+        existing_matched_original_entry_id: set[int] \
+            = {x.id for x in form.obj.entries if len(x.offsets) > 0}
+        entry_id_in_form: set[int] \
+            = {x.eid.data for x in form.entries if x.eid.data is not None}
+        for entry_id in existing_matched_original_entry_id:
+            if entry_id not in entry_id_in_form:
+                raise ValidationError(lazy_gettext(
+                    "Journal entries with offset cannot be deleted."))
+
+
 class TransactionForm(FlaskForm):
     """The base form to create or edit a transaction."""
     date = DateField()
@@ -76,6 +127,19 @@ class TransactionForm(FlaskForm):
         """The journal entry collector.  The default is the base abstract
         collector only to provide the correct type.  The subclass forms should
         provide their own collectors."""
+        self.obj: Transaction | None = kwargs.get("obj")
+        """The transaction, when editing an existing one."""
+        self._is_payable_needed: bool = False
+        """Whether we need the payable original entries."""
+        self._is_receivable_needed: bool = False
+        """Whether we need the receivable original entries."""
+        self.__original_entry_options: list[JournalEntry] | None = None
+        """The options of the original entries."""
+        self.__net_balance_exceeded: dict[int, LazyString] | None = None
+        """The original entries whose net balances were exceeded by the
+        amounts in the journal entry sub-forms."""
+        for entry in self.entries:
+            entry.txn_form = self
 
     def populate_obj(self, obj: Transaction) -> None:
         """Populates the form data into a transaction object.
@@ -86,6 +150,7 @@ class TransactionForm(FlaskForm):
         is_new: bool = obj.id is None
         if is_new:
             obj.id = new_id(Transaction)
+        self.date: DateField
         self.__set_date(obj, self.date.data)
         obj.note = self.note.data
 
@@ -107,8 +172,18 @@ class TransactionForm(FlaskForm):
             obj.created_by_id = current_user_pk
             obj.updated_by_id = current_user_pk
 
-    @staticmethod
+    @property
-    def __set_date(obj: Transaction, new_date: date) -> None:
+    def entries(self) -> list[JournalEntryForm]:
+        """Collects and returns the journal entry sub-forms.
+
+        :return: The journal entry sub-forms.
+        """
+        entries: list[JournalEntryForm] = []
+        for currency in self.currencies:
+            entries.extend(currency.entries)
+        return entries
+
+    def __set_date(self, obj: Transaction, new_date: dt.date) -> None:
         """Sets the transaction date and number.
 
         :param obj: The transaction object.
@@ -118,6 +193,18 @@ class TransactionForm(FlaskForm):
         if obj.date is None or obj.date != new_date:
             if obj.date is not None:
                 sort_transactions_in(obj.date, obj.id)
+            if self.max_date is not None and new_date == self.max_date:
+                db_min_no: int | None = db.session.scalar(
+                    sa.select(sa.func.min(Transaction.no))
+                    .filter(Transaction.date == new_date))
+                if db_min_no is None:
+                    obj.date = new_date
+                    obj.no = 1
+                else:
+                    obj.date = new_date
+                    obj.no = db_min_no - 1
+                    sort_transactions_in(new_date)
+            else:
                 sort_transactions_in(new_date, obj.id)
                 count: int = Transaction.query\
                     .filter(Transaction.date == new_date).count()
@@ -131,7 +218,8 @@ class TransactionForm(FlaskForm):
         :return: The selectable debit accounts.
         """
         accounts: list[AccountOption] \
-            = [AccountOption(x) for x in Account.debit()]
+            = [AccountOption(x) for x in Account.debit()
+               if not (x.code[0] == "2" and x.is_offset_needed)]
         in_use: set[int] = set(db.session.scalars(
             sa.select(JournalEntry.account_id)
             .filter(JournalEntry.is_debit)
@@ -147,7 +235,8 @@ class TransactionForm(FlaskForm):
         :return: The selectable credit accounts.
         """
         accounts: list[AccountOption] \
-            = [AccountOption(x) for x in Account.credit()]
+            = [AccountOption(x) for x in Account.credit()
+               if not (x.code[0] == "1" and x.is_offset_needed)]
         in_use: set[int] = set(db.session.scalars(
             sa.select(JournalEntry.account_id)
             .filter(sa.not_(JournalEntry.is_debit))
@@ -173,6 +262,46 @@ class TransactionForm(FlaskForm):
         """
         return SummaryEditor()
 
+    @property
+    def original_entry_options(self) -> list[JournalEntry]:
+        """Returns the selectable original entries.
+
+        :return: The selectable original entries.
+        """
+        if self.__original_entry_options is None:
+            self.__original_entry_options = get_selectable_original_entries(
+                {x.eid.data for x in self.entries if x.eid.data is not None},
+                self._is_payable_needed, self._is_receivable_needed)
+        return self.__original_entry_options
+
+    @property
+    def min_date(self) -> dt.date | None:
+        """Returns the minimal available date.
+
+        :return: The minimal available date.
+        """
+        original_entry_id: set[int] \
+            = {x.original_entry_id.data for x in self.entries
+               if x.original_entry_id.data is not None}
+        if len(original_entry_id) == 0:
+            return None
+        select: sa.Select = sa.select(sa.func.max(Transaction.date))\
+            .join(JournalEntry).filter(JournalEntry.id.in_(original_entry_id))
+        return db.session.scalar(select)
+
+    @property
+    def max_date(self) -> dt.date | None:
+        """Returns the maximum available date.
+
+        :return: The maximum available date.
+        """
+        entry_id: set[int] = {x.eid.data for x in self.entries
+                              if x.eid.data is not None}
+        select: sa.Select = sa.select(sa.func.min(Transaction.date))\
+            .join(JournalEntry)\
+            .filter(JournalEntry.original_entry_id.in_(entry_id))
+        return db.session.scalar(select)
+
 
 T = t.TypeVar("T", bound=TransactionForm)
 """A transaction form variant."""
@@ -314,16 +443,23 @@ class JournalEntryCollector(t.Generic[T], ABC):
 
 class IncomeTransactionForm(TransactionForm):
     """The form to create or edit a cash income transaction."""
-    date = DateField(validators=[DATE_REQUIRED])
+    date = DateField(
+        validators=[DATE_REQUIRED,
+                    NotBeforeOriginalEntries(),
+                    NotAfterOffsetEntries()])
     """The date."""
     currencies = FieldList(FormField(IncomeCurrencyForm), name="currency",
                            validators=[NeedSomeCurrencies()])
     """The journal entries categorized by their currencies."""
     note = TextAreaField(filters=[strip_multiline_text])
     """The note."""
+    whole_form = BooleanField(
+        validators=[CannotDeleteOriginalEntriesWithOffset()])
+    """The pseudo field for the whole form validators."""
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
+        self._is_receivable_needed = True
 
         class Collector(JournalEntryCollector[IncomeTransactionForm]):
             """The journal entry collector for the cash income transactions."""
@@ -352,16 +488,23 @@ class IncomeTransactionForm(TransactionForm):
 
 class ExpenseTransactionForm(TransactionForm):
     """The form to create or edit a cash expense transaction."""
-    date = DateField(validators=[DATE_REQUIRED])
+    date = DateField(
+        validators=[DATE_REQUIRED,
+                    NotBeforeOriginalEntries(),
+                    NotAfterOffsetEntries()])
     """The date."""
     currencies = FieldList(FormField(ExpenseCurrencyForm), name="currency",
                            validators=[NeedSomeCurrencies()])
     """The journal entries categorized by their currencies."""
     note = TextAreaField(filters=[strip_multiline_text])
     """The note."""
+    whole_form = BooleanField(
+        validators=[CannotDeleteOriginalEntriesWithOffset()])
+    """The pseudo field for the whole form validators."""
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
+        self._is_payable_needed = True
 
         class Collector(JournalEntryCollector[ExpenseTransactionForm]):
             """The journal entry collector for the cash expense
@@ -391,16 +534,24 @@ class ExpenseTransactionForm(TransactionForm):
 
 class TransferTransactionForm(TransactionForm):
     """The form to create or edit a transfer transaction."""
-    date = DateField(validators=[DATE_REQUIRED])
+    date = DateField(
+        validators=[DATE_REQUIRED,
+                    NotBeforeOriginalEntries(),
+                    NotAfterOffsetEntries()])
     """The date."""
     currencies = FieldList(FormField(TransferCurrencyForm), name="currency",
                            validators=[NeedSomeCurrencies()])
     """The journal entries categorized by their currencies."""
     note = TextAreaField(filters=[strip_multiline_text])
     """The note."""
+    whole_form = BooleanField(
+        validators=[CannotDeleteOriginalEntriesWithOffset()])
+    """The pseudo field for the whole form validators."""
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
+        self._is_payable_needed = True
+        self._is_receivable_needed = True
 
         class Collector(JournalEntryCollector[TransferTransactionForm]):
             """The journal entry collector for the transfer transactions."""

src/accounting/transaction/utils/account_option.py

@@ -38,6 +38,10 @@ class AccountOption:
         """The string representation of the account option."""
         self.is_in_use: bool = False
         """True if this account is in use, or False otherwise."""
+        self.is_offset_needed: bool = account.is_offset_needed
+        """True if this account needs offset, or False otherwise."""
+        self.is_offset_chooser_needed: bool = False
+        """True if this account needs an offset chooser, or False otherwise."""
 
     def __str__(self) -> str:
         """Returns the string representation of the account option.

src/accounting/transaction/utils/offset_alias.py

@@ -0,0 +1,39 @@
+# The Mia! Accounting Flask Project.
+# Author: imacat@mail.imacat.idv.tw (imacat), 2023/3/15
+
+#  Copyright (c) 2023 imacat.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+"""The SQLAlchemy alias for the offset entries.
+
+"""
+import typing as t
+
+import sqlalchemy as sa
+
+from accounting.models import JournalEntry
+
+
+def offset_alias() -> sa.Alias:
+    """Returns the SQLAlchemy alias for the offset entries.
+
+    :return: The SQLAlchemy alias for the offset entries.
+    """
+
+    def as_from(model_cls: t.Any) -> sa.FromClause:
+        return model_cls
+
+    def as_alias(alias: t.Any) -> sa.Alias:
+        return alias
+
+    return as_alias(sa.alias(as_from(JournalEntry), name="offset"))

src/accounting/transaction/utils/original_entries.py

@@ -0,0 +1,82 @@
+# The Mia! Accounting Flask Project.
+# Author: imacat@mail.imacat.idv.tw (imacat), 2023/3/10
+
+#  Copyright (c) 2023 imacat.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+"""The selectable original entries.
+
+"""
+from decimal import Decimal
+
+import sqlalchemy as sa
+from flask_babel import LazyString
+from sqlalchemy.orm import selectinload
+
+from accounting import db
+from accounting.locale import lazy_gettext
+from accounting.models import Account, Transaction, JournalEntry
+from accounting.transaction.forms.journal_entry import JournalEntryForm
+from accounting.utils.cast import be
+from .offset_alias import offset_alias
+
+
+def get_selectable_original_entries(
+        entry_id_on_form: set[int], is_payable: bool, is_receivable: bool) \
+        -> list[JournalEntry]:
+    """Queries and returns the selectable original entries, with their net
+    balances.  The offset amounts of the form is excluded.
+
+    :param entry_id_on_form: The ID of the journal entries on the form.
+    :param is_payable: True to check the payable original entries, or False
+        otherwise.
+    :param is_receivable: True to check the receivable original entries, or
+        False otherwise.
+    :return: The selectable original entries, with their net balances.
+    """
+    assert is_payable or is_receivable
+    offset: sa.Alias = offset_alias()
+    net_balance: sa.Label = (JournalEntry.amount + sa.func.sum(sa.case(
+        (offset.c.id.in_(entry_id_on_form), 0),
+        (be(offset.c.is_debit == JournalEntry.is_debit), offset.c.amount),
+        else_=-offset.c.amount))).label("net_balance")
+    conditions: list[sa.BinaryExpression] = [Account.is_offset_needed]
+    sub_conditions: list[sa.BinaryExpression] = []
+    if is_payable:
+        sub_conditions.append(sa.and_(Account.base_code.startswith("2"),
+                                      sa.not_(JournalEntry.is_debit)))
+    if is_receivable:
+        sub_conditions.append(sa.and_(Account.base_code.startswith("1"),
+                                      JournalEntry.is_debit))
+    conditions.append(sa.or_(*sub_conditions))
+    select_net_balances: sa.Select = sa.select(JournalEntry.id, net_balance)\
+        .join(Account)\
+        .join(offset, be(JournalEntry.id == offset.c.original_entry_id),
+              isouter=True)\
+        .filter(*conditions)\
+        .group_by(JournalEntry.id)\
+        .having(sa.or_(sa.func.count(offset.c.id) == 0, net_balance != 0))
+    net_balances: dict[int, Decimal] \
+        = {x.id: x.net_balance
+           for x in db.session.execute(select_net_balances).all()}
+    entries: list[JournalEntry] = JournalEntry.query\
+        .filter(JournalEntry.id.in_({x for x in net_balances}))\
+        .join(Transaction)\
+        .order_by(Transaction.date, JournalEntry.is_debit, JournalEntry.no)\
+        .options(selectinload(JournalEntry.currency),
+                 selectinload(JournalEntry.account),
+                 selectinload(JournalEntry.transaction)).all()
+    for entry in entries:
+        entry.net_balance = entry.amount if net_balances[entry.id] is None \
+            else net_balances[entry.id]
+    return entries

src/accounting/transaction/utils/summary_editor.py

@@ -218,7 +218,8 @@ class SummaryEditor:
                                       JournalEntry.account_id,
                                       sa.func.count().label("freq"))\
             .filter(JournalEntry.summary.is_not(None),
-                    JournalEntry.summary.like("_%—_%"))\
+                    JournalEntry.summary.like("_%—_%"),
+                    JournalEntry.original_entry_id.is_(None))\
             .group_by(entry_type, tag_type, tag, JournalEntry.account_id)
         result: list[sa.Row] = db.session.execute(select).all()
         accounts: dict[int, Account] \

src/accounting/transaction/views.py

@@ -117,6 +117,7 @@ def show_transaction_edit_form(txn: Transaction) -> str:
     if "form" in session:
         form = txn_op.form(ImmutableMultiDict(parse_qsl(session["form"])))
         del session["form"]
+        form.obj = txn
         form.validate()
     else:
         form = txn_op.form(obj=txn)
@@ -134,6 +135,7 @@ def update_transaction(txn: Transaction) -> redirect:
     """
     txn_op: TransactionOperator = get_txn_op(txn, is_check_as=True)
     form: txn_op.form = txn_op.form(request.form)
+    form.obj = txn
     if not form.validate():
         flash_form_errors(form)
         session["form"] = urlencode(list(request.form.items()))

tests/test_offset.py

@@ -0,0 +1,656 @@
+# The Mia! Accounting Flask Project.
+# Author: imacat@mail.imacat.idv.tw (imacat), 2023/3/11
+
+#  Copyright (c) 2023 imacat.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+"""The test for the offset.
+
+"""
+import unittest
+
+import httpx
+from click.testing import Result
+from flask import Flask
+from flask.testing import FlaskCliRunner
+
+from test_site import db
+from testlib import create_test_app, get_client
+from testlib_offset import TestData, JournalEntryData, TransactionData, \
+    CurrencyData
+from testlib_txn import Accounts, match_txn_detail
+
+PREFIX: str = "/accounting/transactions"
+"""The URL prefix for the transaction management."""
+
+
+class OffsetTestCase(unittest.TestCase):
+    """The offset test case."""
+
+    def setUp(self) -> None:
+        """Sets up the test.
+        This is run once per test.
+
+        :return: None.
+        """
+        self.app: Flask = create_test_app()
+
+        runner: FlaskCliRunner = self.app.test_cli_runner()
+        with self.app.app_context():
+            from accounting.models import BaseAccount, Transaction, \
+                JournalEntry
+            result: Result
+            result = runner.invoke(args="init-db")
+            self.assertEqual(result.exit_code, 0)
+            if BaseAccount.query.first() is None:
+                result = runner.invoke(args="accounting-init-base")
+                self.assertEqual(result.exit_code, 0)
+            result = runner.invoke(args=["accounting-init-currencies",
+                                         "-u", "editor"])
+            self.assertEqual(result.exit_code, 0)
+            result = runner.invoke(args=["accounting-init-accounts",
+                                         "-u", "editor"])
+            self.assertEqual(result.exit_code, 0)
+            Transaction.query.delete()
+            JournalEntry.query.delete()
+
+        self.client, self.csrf_token = get_client(self.app, "editor")
+        self.data: TestData = TestData(self.app, self.client, self.csrf_token)
+
+    def test_add_receivable_offset(self) -> None:
+        """Tests to add the receivable offset.
+
+        :return: None.
+        """
+        from accounting.models import Account, Transaction
+        create_uri: str = f"{PREFIX}/create/income?next=%2F_next"
+        store_uri: str = f"{PREFIX}/store/income"
+        form: dict[str, str]
+        response: httpx.Response
+
+        txn_data: TransactionData = TransactionData(
+            self.data.e_r_or3d.txn.days, [CurrencyData(
+                "USD",
+                [],
+                [JournalEntryData(Accounts.RECEIVABLE,
+                                  self.data.e_r_or1d.summary, "300",
+                                  original_entry=self.data.e_r_or1d),
+                 JournalEntryData(Accounts.RECEIVABLE,
+                                  self.data.e_r_or1d.summary, "100",
+                                  original_entry=self.data.e_r_or1d),
+                 JournalEntryData(Accounts.RECEIVABLE,
+                                  self.data.e_r_or3d.summary, "100",
+                                  original_entry=self.data.e_r_or3d)])])
+
+        # Non-existing original entry ID
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-credit-1-original_entry_id"] = "9999"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # The same side
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-credit-1-original_entry_id"] = self.data.e_p_or1c.id
+        form["currency-1-credit-1-account_code"] = self.data.e_p_or1c.account
+        form["currency-1-credit-1-amount"] = "100"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # The original entry does not need offset
+        with self.app.app_context():
+            account = Account.find_by_code(Accounts.RECEIVABLE)
+            account.is_offset_needed = False
+            db.session.commit()
+        response = self.client.post(store_uri,
+                                    data=txn_data.new_form(self.csrf_token))
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+        with self.app.app_context():
+            account = Account.find_by_code(Accounts.RECEIVABLE)
+            account.is_offset_needed = True
+            db.session.commit()
+
+        # The original entry is also an offset
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-credit-1-original_entry_id"] = self.data.e_p_of1d.id
+        form["currency-1-credit-1-account_code"] = self.data.e_p_of1d.account
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not the same currency
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-code"] = "EUR"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not the same account
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-credit-1-account_code"] = Accounts.NOTES_RECEIVABLE
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not exceeding net balance - partially offset
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-credit-1-amount"] = "300.01"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not exceeding net balance - unmatched
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-credit-3-amount"] = "100.01"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not before the original entries
+        txn_data.days = self.data.e_r_or3d.txn.days + 1
+        form = txn_data.new_form(self.csrf_token)
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+        txn_data.days = 0
+
+        # Success
+        form = txn_data.new_form(self.csrf_token)
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        txn_id: int = match_txn_detail(response.headers["Location"])
+        with self.app.app_context():
+            txn = db.session.get(Transaction, txn_id)
+            for offset in txn.currencies[0].credit:
+                self.assertIsNotNone(offset.original_entry_id)
+
+    def test_edit_receivable_offset(self) -> None:
+        """Tests to edit the receivable offset.
+
+        :return: None.
+        """
+        from accounting.models import Account
+        txn_data: TransactionData = self.data.t_r_of2
+        edit_uri: str = f"{PREFIX}/{txn_data.id}/edit?next=%2F_next"
+        update_uri: str = f"{PREFIX}/{txn_data.id}/update"
+        form: dict[str, str]
+        response: httpx.Response
+
+        # Non-existing original entry ID
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-credit-1-original_entry_id"] = "9999"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # The same side
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-credit-1-original_entry_id"] = self.data.e_p_or1c.id
+        form["currency-1-credit-1-account_code"] = self.data.e_p_or1c.account
+        form["currency-1-debit-1-amount"] = "100"
+        form["currency-1-credit-1-amount"] = "100"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # The original entry does not need offset
+        with self.app.app_context():
+            account = Account.find_by_code(Accounts.RECEIVABLE)
+            account.is_offset_needed = False
+            db.session.commit()
+        response = self.client.post(update_uri,
+                                    data=txn_data.update_form(self.csrf_token))
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+        with self.app.app_context():
+            account = Account.find_by_code(Accounts.RECEIVABLE)
+            account.is_offset_needed = True
+            db.session.commit()
+
+        # The original entry is also an offset
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-credit-1-original_entry_id"] = self.data.e_p_of1d.id
+        form["currency-1-credit-1-account_code"] = self.data.e_p_of1d.account
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not the same currency
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-code"] = "EUR"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not the same account
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-credit-1-account_code"] = Accounts.NOTES_RECEIVABLE
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not exceeding net balance - partially offset
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-amount"] = "600.01"
+        form["currency-1-credit-1-amount"] = "600.01"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not exceeding net balance - unmatched
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-3-amount"] = "600.01"
+        form["currency-1-credit-3-amount"] = "600.01"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not before the original entries
+        txn_data.days = self.data.e_r_or3d.txn.days + 1
+        form = txn_data.update_form(self.csrf_token)
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+        txn_data.days = 0
+
+        # Success
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-amount"] = "600"
+        form["currency-1-credit-1-amount"] = "600"
+        form["currency-1-debit-3-amount"] = "600"
+        form["currency-1-credit-3-amount"] = "600"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"],
+                         f"{PREFIX}/{txn_data.id}?next=%2F_next")
+
+    def test_edit_receivable_original_entry(self) -> None:
+        """Tests to edit the receivable original entry.
+
+        :return: None.
+        """
+        from accounting.models import Transaction
+        txn_data: TransactionData = self.data.t_r_or1
+        edit_uri: str = f"{PREFIX}/{txn_data.id}/edit?next=%2F_next"
+        update_uri: str = f"{PREFIX}/{txn_data.id}/update"
+        form: dict[str, str]
+        response: httpx.Response
+
+        txn_data.days = self.data.t_r_of1.days
+        txn_data.currencies[0].debit[0].amount = "800"
+        txn_data.currencies[0].credit[0].amount = "800"
+        txn_data.currencies[0].debit[1].amount = "3.4"
+        txn_data.currencies[0].credit[1].amount = "3.4"
+
+        # Not the same currency
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-code"] = "EUR"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not the same account
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-account_code"] = Accounts.NOTES_RECEIVABLE
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not less than offset total - partially offset
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-amount"] = "799.99"
+        form["currency-1-credit-1-amount"] = "799.99"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not less than offset total - fully offset
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-2-amount"] = "3.39"
+        form["currency-1-credit-2-amount"] = "3.39"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not after the offset entries
+        old_days: int = txn_data.days
+        txn_data.days = self.data.t_r_of1.days - 1
+        form = txn_data.update_form(self.csrf_token)
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+        txn_data.days = old_days
+
+        # Not deleting matched original entries
+        form = txn_data.update_form(self.csrf_token)
+        del form["currency-1-debit-1-eid"]
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Success
+        form = txn_data.update_form(self.csrf_token)
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"],
+                         f"{PREFIX}/{txn_data.id}?next=%2F_next")
+
+        # The original entry is always before the offset entry, even when they
+        # happen in the same day.
+        with self.app.app_context():
+            txn_or: Transaction | None = db.session.get(
+                Transaction, txn_data.id)
+            self.assertIsNotNone(txn_or)
+            txn_of: Transaction | None = db.session.get(
+                Transaction, self.data.t_r_of1.id)
+            self.assertIsNotNone(txn_of)
+            self.assertEqual(txn_or.date, txn_of.date)
+            self.assertLess(txn_or.no, txn_of.no)
+
+    def test_add_payable_offset(self) -> None:
+        """Tests to add the payable offset.
+
+        :return: None.
+        """
+        from accounting.models import Account, Transaction
+        create_uri: str = f"{PREFIX}/create/expense?next=%2F_next"
+        store_uri: str = f"{PREFIX}/store/expense"
+        form: dict[str, str]
+        response: httpx.Response
+
+        txn_data: TransactionData = TransactionData(
+            self.data.e_p_or3c.txn.days, [CurrencyData(
+                "USD",
+                [JournalEntryData(Accounts.PAYABLE,
+                                  self.data.e_p_or1c.summary, "500",
+                                  original_entry=self.data.e_p_or1c),
+                 JournalEntryData(Accounts.PAYABLE,
+                                  self.data.e_p_or1c.summary, "300",
+                                  original_entry=self.data.e_p_or1c),
+                 JournalEntryData(Accounts.PAYABLE,
+                                  self.data.e_p_or3c.summary, "120",
+                                  original_entry=self.data.e_p_or3c)],
+                [])])
+
+        # Non-existing original entry ID
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-debit-1-original_entry_id"] = "9999"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # The same side
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-debit-1-original_entry_id"] = self.data.e_r_or1d.id
+        form["currency-1-debit-1-account_code"] = self.data.e_r_or1d.account
+        form["currency-1-debit-1-amount"] = "100"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # The original entry does not need offset
+        with self.app.app_context():
+            account = Account.find_by_code(Accounts.PAYABLE)
+            account.is_offset_needed = False
+            db.session.commit()
+        response = self.client.post(store_uri,
+                                    data=txn_data.new_form(self.csrf_token))
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+        with self.app.app_context():
+            account = Account.find_by_code(Accounts.PAYABLE)
+            account.is_offset_needed = True
+            db.session.commit()
+
+        # The original entry is also an offset
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-debit-1-original_entry_id"] = self.data.e_r_of1c.id
+        form["currency-1-debit-1-account_code"] = self.data.e_r_of1c.account
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not the same currency
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-code"] = "EUR"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not the same account
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-debit-1-account_code"] = Accounts.NOTES_PAYABLE
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not exceeding net balance - partially offset
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-debit-1-amount"] = "500.01"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not exceeding net balance - unmatched
+        form = txn_data.new_form(self.csrf_token)
+        form["currency-1-debit-3-amount"] = "120.01"
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # Not before the original entries
+        txn_data.days = self.data.e_p_or3c.txn.days + 1
+        form = txn_data.new_form(self.csrf_token)
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+        txn_data.days = 0
+
+        # Success
+        form = txn_data.new_form(self.csrf_token)
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        txn_id: int = match_txn_detail(response.headers["Location"])
+        with self.app.app_context():
+            txn = db.session.get(Transaction, txn_id)
+            for offset in txn.currencies[0].debit:
+                self.assertIsNotNone(offset.original_entry_id)
+
+    def test_edit_payable_offset(self) -> None:
+        """Tests to edit the payable offset.
+
+        :return: None.
+        """
+        from accounting.models import Account, Transaction
+        txn_data: TransactionData = self.data.t_p_of2
+        edit_uri: str = f"{PREFIX}/{txn_data.id}/edit?next=%2F_next"
+        update_uri: str = f"{PREFIX}/{txn_data.id}/update"
+        form: dict[str, str]
+        response: httpx.Response
+
+        # Non-existing original entry ID
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-original_entry_id"] = "9999"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # The same side
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-original_entry_id"] = self.data.e_r_or1d.id
+        form["currency-1-debit-1-account_code"] = self.data.e_r_or1d.account
+        form["currency-1-debit-1-amount"] = "100"
+        form["currency-1-credit-1-amount"] = "100"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # The original entry does not need offset
+        with self.app.app_context():
+            account = Account.find_by_code(Accounts.PAYABLE)
+            account.is_offset_needed = False
+            db.session.commit()
+        response = self.client.post(update_uri,
+                                    data=txn_data.update_form(self.csrf_token))
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+        with self.app.app_context():
+            account = Account.find_by_code(Accounts.PAYABLE)
+            account.is_offset_needed = True
+            db.session.commit()
+
+        # The original entry is also an offset
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-original_entry_id"] = self.data.e_r_of1c.id
+        form["currency-1-debit-1-account_code"] = self.data.e_r_of1c.account
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not the same currency
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-code"] = "EUR"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not the same account
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-account_code"] = Accounts.NOTES_PAYABLE
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not exceeding net balance - partially offset
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-amount"] = "1100.01"
+        form["currency-1-credit-1-amount"] = "1100.01"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not exceeding net balance - unmatched
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-3-amount"] = "900.01"
+        form["currency-1-credit-3-amount"] = "900.01"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not before the original entries
+        old_days: int = txn_data.days
+        txn_data.days = self.data.e_p_or3c.txn.days + 1
+        form = txn_data.update_form(self.csrf_token)
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+        txn_data.days = old_days
+
+        # Success
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-amount"] = "1100"
+        form["currency-1-credit-1-amount"] = "1100"
+        form["currency-1-debit-3-amount"] = "900"
+        form["currency-1-credit-3-amount"] = "900"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        txn_id: int = match_txn_detail(response.headers["Location"])
+        with self.app.app_context():
+            txn = db.session.get(Transaction, txn_id)
+            for offset in txn.currencies[0].debit:
+                self.assertIsNotNone(offset.original_entry_id)
+
+    def test_edit_payable_original_entry(self) -> None:
+        """Tests to edit the payable original entry.
+
+        :return: None.
+        """
+        from accounting.models import Transaction
+        txn_data: TransactionData = self.data.t_p_or1
+        edit_uri: str = f"{PREFIX}/{txn_data.id}/edit?next=%2F_next"
+        update_uri: str = f"{PREFIX}/{txn_data.id}/update"
+        form: dict[str, str]
+        response: httpx.Response
+
+        txn_data.days = self.data.t_p_of1.days
+        txn_data.currencies[0].debit[0].amount = "1200"
+        txn_data.currencies[0].credit[0].amount = "1200"
+        txn_data.currencies[0].debit[1].amount = "0.9"
+        txn_data.currencies[0].credit[1].amount = "0.9"
+
+        # Not the same currency
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-code"] = "EUR"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not the same account
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-credit-1-account_code"] = Accounts.NOTES_PAYABLE
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not less than offset total - partially offset
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-1-amount"] = "1199.99"
+        form["currency-1-credit-1-amount"] = "1199.99"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not less than offset total - fully offset
+        form = txn_data.update_form(self.csrf_token)
+        form["currency-1-debit-2-amount"] = "0.89"
+        form["currency-1-credit-2-amount"] = "0.89"
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Not after the offset entries
+        old_days: int = txn_data.days
+        txn_data.days = self.data.t_p_of1.days - 1
+        form = txn_data.update_form(self.csrf_token)
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+        txn_data.days = old_days
+
+        # Not deleting matched original entries
+        form = txn_data.update_form(self.csrf_token)
+        del form["currency-1-credit-1-eid"]
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # Success
+        form = txn_data.update_form(self.csrf_token)
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"],
+                         f"{PREFIX}/{txn_data.id}?next=%2F_next")
+
+        # The original entry is always before the offset entry, even when they
+        # happen in the same day
+        with self.app.app_context():
+            txn_or: Transaction | None = db.session.get(
+                Transaction, txn_data.id)
+            self.assertIsNotNone(txn_or)
+            txn_of: Transaction | None = db.session.get(
+                Transaction, self.data.t_p_of1.id)
+            self.assertIsNotNone(txn_of)
+            self.assertEqual(txn_or.date, txn_of.date)
+            self.assertLess(txn_or.no, txn_of.no)

tests/test_transaction.py

@@ -229,6 +229,15 @@ class CashIncomeTransactionTestCase(unittest.TestCase):
         self.assertEqual(response.status_code, 302)
         self.assertEqual(response.headers["Location"], create_uri)
 
+        # A receivable entry cannot start from the credit side
+        form = self.__get_add_form()
+        key: str = [x for x in form.keys()
+                    if x.endswith("-account_code") and "-credit-" in x][0]
+        form[key] = Accounts.RECEIVABLE
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
         # Negative amount
         form = self.__get_add_form()
         set_negative_amount(form)
@@ -380,6 +389,15 @@ class CashIncomeTransactionTestCase(unittest.TestCase):
         self.assertEqual(response.status_code, 302)
         self.assertEqual(response.headers["Location"], edit_uri)
 
+        # A receivable entry cannot start from the credit side
+        form = self.__get_add_form()
+        key: str = [x for x in form.keys()
+                    if x.endswith("-account_code") and "-credit-" in x][0]
+        form[key] = Accounts.RECEIVABLE
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
         # Negative amount
         form: dict[str, str] = form_0.copy()
         set_negative_amount(form)
@@ -781,6 +799,15 @@ class CashExpenseTransactionTestCase(unittest.TestCase):
         self.assertEqual(response.status_code, 302)
         self.assertEqual(response.headers["Location"], create_uri)
 
+        # A payable entry cannot start from the debit side
+        form = self.__get_add_form()
+        key: str = [x for x in form.keys()
+                    if x.endswith("-account_code") and "-debit-" in x][0]
+        form[key] = Accounts.PAYABLE
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
         # Negative amount
         form = self.__get_add_form()
         set_negative_amount(form)
@@ -935,6 +962,15 @@ class CashExpenseTransactionTestCase(unittest.TestCase):
         self.assertEqual(response.status_code, 302)
         self.assertEqual(response.headers["Location"], edit_uri)
 
+        # A payable entry cannot start from the debit side
+        form = self.__get_add_form()
+        key: str = [x for x in form.keys()
+                    if x.endswith("-account_code") and "-debit-" in x][0]
+        form[key] = Accounts.PAYABLE
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
         # Negative amount
         form: dict[str, str] = form_0.copy()
         set_negative_amount(form)
@@ -1356,6 +1392,24 @@ class TransferTransactionTestCase(unittest.TestCase):
         self.assertEqual(response.status_code, 302)
         self.assertEqual(response.headers["Location"], create_uri)
 
+        # A receivable entry cannot start from the credit side
+        form = self.__get_add_form()
+        key: str = [x for x in form.keys()
+                    if x.endswith("-account_code") and "-credit-" in x][0]
+        form[key] = Accounts.RECEIVABLE
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
+        # A payable entry cannot start from the debit side
+        form = self.__get_add_form()
+        key: str = [x for x in form.keys()
+                    if x.endswith("-account_code") and "-debit-" in x][0]
+        form[key] = Accounts.PAYABLE
+        response = self.client.post(store_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], create_uri)
+
         # Negative amount
         form = self.__get_add_form()
         set_negative_amount(form)
@@ -1537,6 +1591,24 @@ class TransferTransactionTestCase(unittest.TestCase):
         self.assertEqual(response.status_code, 302)
         self.assertEqual(response.headers["Location"], edit_uri)
 
+        # A receivable entry cannot start from the credit side
+        form = self.__get_add_form()
+        key: str = [x for x in form.keys()
+                    if x.endswith("-account_code") and "-credit-" in x][0]
+        form[key] = Accounts.RECEIVABLE
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
+        # A payable entry cannot start from the debit side
+        form = self.__get_add_form()
+        key: str = [x for x in form.keys()
+                    if x.endswith("-account_code") and "-debit-" in x][0]
+        form[key] = Accounts.PAYABLE
+        response = self.client.post(update_uri, data=form)
+        self.assertEqual(response.status_code, 302)
+        self.assertEqual(response.headers["Location"], edit_uri)
+
         # Negative amount
         form: dict[str, str] = form_0.copy()
         set_negative_amount(form)

tests/testlib_offset.py

@@ -0,0 +1,307 @@
+# The Mia! Accounting Flask Project.
+# Author: imacat@mail.imacat.idv.tw (imacat), 2023/2/27
+
+#  Copyright (c) 2023 imacat.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+"""The common test libraries for the offset test cases.
+
+"""
+from __future__ import annotations
+from datetime import date, timedelta
+
+import httpx
+from flask import Flask
+
+from test_site import db
+from testlib_txn import Accounts, match_txn_detail, NEXT_URI
+
+
+class JournalEntryData:
+    """The journal entry data."""
+
+    def __init__(self, account: str, summary: str, amount: str,
+                 original_entry: JournalEntryData | None = None):
+        """Constructs the journal entry data.
+
+        :param account: The account code.
+        :param summary: The summary.
+        :param amount: The amount.
+        :param original_entry: The original entry.
+        """
+        self.txn: TransactionData | None = None
+        self.id: int = -1
+        self.no: int = -1
+        self.original_entry: JournalEntryData | None = original_entry
+        self.account: str = account
+        self.summary: str = summary
+        self.amount: str = amount
+
+    def form(self, prefix: str, entry_type: str, index: int, is_update: bool) \
+            -> dict[str, str]:
+        """Returns the journal entry as form data.
+
+        :param prefix: The prefix of the form fields.
+        :param entry_type: The entry type, either "debit" or "credit".
+        :param index: The entry index.
+        :param is_update: True for an update operation, or False otherwise
+        :return: The form data.
+        """
+        prefix = f"{prefix}-{entry_type}-{index}"
+        form: dict[str, str] = {f"{prefix}-account_code": self.account,
+                                f"{prefix}-summary": self.summary,
+                                f"{prefix}-amount": self.amount}
+        if is_update and self.id != -1:
+            form[f"{prefix}-eid"] = str(self.id)
+        form[f"{prefix}-no"] = str(index) if self.no == -1 else str(self.no)
+        if self.original_entry is not None:
+            assert self.original_entry.id != -1
+            form[f"{prefix}-original_entry_id"] = str(self.original_entry.id)
+        return form
+
+
+class CurrencyData:
+    """The transaction currency data."""
+
+    def __init__(self, currency: str, debit: list[JournalEntryData],
+                 credit: list[JournalEntryData]):
+        """Constructs the transaction currency data.
+
+        :param currency: The currency code.
+        :param debit: The debit journal entries.
+        :param credit: The credit journal entries.
+        """
+        self.code: str = currency
+        self.debit: list[JournalEntryData] = debit
+        self.credit: list[JournalEntryData] = credit
+
+    def form(self, index: int, is_update: bool) -> dict[str, str]:
+        """Returns the currency as form data.
+
+        :param index: The currency index.
+        :param is_update: True for an update operation, or False otherwise
+        :return: The form data.
+        """
+        prefix: str = f"currency-{index}"
+        form: dict[str, str] = {f"{prefix}-code": self.code}
+        for i in range(len(self.debit)):
+            form.update(self.debit[i].form(prefix, "debit", i + 1, is_update))
+        for i in range(len(self.credit)):
+            form.update(self.credit[i].form(prefix, "credit", i + 1,
+                                            is_update))
+        return form
+
+
+class TransactionData:
+    """The transaction data."""
+
+    def __init__(self, days: int, currencies: list[CurrencyData]):
+        """Constructs a transaction.
+
+        :param days: The number of days before today.
+        :param currencies: The transaction currency data.
+        """
+        self.id: int = -1
+        self.days: int = days
+        self.currencies: list[CurrencyData] = currencies
+        self.note: str | None = None
+        for currency in self.currencies:
+            for entry in currency.debit:
+                entry.txn = self
+            for entry in currency.credit:
+                entry.txn = self
+
+    def new_form(self, csrf_token: str) -> dict[str, str]:
+        """Returns the transaction as a form.
+
+        :param csrf_token: The CSRF token.
+        :return: The transaction as a form.
+        """
+        return self.__form(csrf_token, is_update=False)
+
+    def update_form(self, csrf_token: str) -> dict[str, str]:
+        """Returns the transaction as a form.
+
+        :param csrf_token: The CSRF token.
+        :return: The transaction as a form.
+        """
+        return self.__form(csrf_token, is_update=True)
+
+    def __form(self, csrf_token: str, is_update: bool = False) \
+            -> dict[str, str]:
+        """Returns the transaction as a form.
+
+        :param csrf_token: The CSRF token.
+        :param is_update: True for an update operation, or False otherwise
+        :return: The transaction as a form.
+        """
+        txn_date: date = date.today() - timedelta(days=self.days)
+        form: dict[str, str] = {"csrf_token": csrf_token,
+                                "next": NEXT_URI,
+                                "date": txn_date.isoformat()}
+        for i in range(len(self.currencies)):
+            form.update(self.currencies[i].form(i + 1, is_update))
+        if self.note is not None:
+            form["note"] = self.note
+        return form
+
+
+class TestData:
+    """The test data."""
+
+    def __init__(self, app: Flask, client: httpx.Client, csrf_token: str):
+        """Constructs the test data.
+
+        :param app: The Flask application.
+        :param client: The client.
+        :param csrf_token: The CSRF token.
+        """
+        self.app: Flask = app
+        self.client: httpx.Client = client
+        self.csrf_token: str = csrf_token
+
+        def couple(summary: str, amount: str, debit: str, credit: str) \
+                -> tuple[JournalEntryData, JournalEntryData]:
+            """Returns a couple of debit-credit journal entries.
+
+            :param summary: The summary.
+            :param amount: The amount.
+            :param debit: The debit account code.
+            :param credit: The credit account code.
+            :return: The debit journal entry and credit journal entry.
+            """
+            return JournalEntryData(debit, summary, amount),\
+                JournalEntryData(credit, summary, amount)
+
+        # Receivable original entries
+        self.e_r_or1d, self.e_r_or1c = couple(
+            "Accountant", "1200", Accounts.RECEIVABLE, Accounts.SERVICE)
+        self.e_r_or2d, self.e_r_or2c = couple(
+            "Toy", "600", Accounts.RECEIVABLE, Accounts.SALES)
+        self.e_r_or3d, self.e_r_or3c = couple(
+            "Noodles", "100", Accounts.RECEIVABLE, Accounts.SALES)
+        self.e_r_or4d, self.e_r_or4c = couple(
+            "Interest", "3.4", Accounts.RECEIVABLE, Accounts.INTEREST)
+
+        # Payable original entries
+        self.e_p_or1d, self.e_p_or1c = couple(
+            "Airplane ticket", "2000", Accounts.TRAVEL, Accounts.PAYABLE)
+        self.e_p_or2d, self.e_p_or2c = couple(
+            "Phone", "900", Accounts.OFFICE, Accounts.PAYABLE)
+        self.e_p_or3d, self.e_p_or3c = couple(
+            "Steak", "120", Accounts.MEAL, Accounts.PAYABLE)
+        self.e_p_or4d, self.e_p_or4c = couple(
+            "Envelop", "0.9", Accounts.OFFICE, Accounts.PAYABLE)
+
+        # Original transactions
+        self.t_r_or1: TransactionData = TransactionData(
+            50, [CurrencyData("USD", [self.e_r_or1d, self.e_r_or4d],
+                              [self.e_r_or1c, self.e_r_or4c])])
+        self.t_r_or2: TransactionData = TransactionData(
+            30, [CurrencyData("USD", [self.e_r_or2d, self.e_r_or3d],
+                              [self.e_r_or2c, self.e_r_or3c])])
+        self.t_p_or1: TransactionData = TransactionData(
+            40, [CurrencyData("USD", [self.e_p_or1d, self.e_p_or4d],
+                              [self.e_p_or1c, self.e_p_or4c])])
+        self.t_p_or2: TransactionData = TransactionData(
+            20, [CurrencyData("USD", [self.e_p_or2d, self.e_p_or3d],
+                              [self.e_p_or2c, self.e_p_or3c])])
+
+        self.__add_txn(self.t_r_or1)
+        self.__add_txn(self.t_r_or2)
+        self.__add_txn(self.t_p_or1)
+        self.__add_txn(self.t_p_or2)
+
+        # Receivable offset entries
+        self.e_r_of1d, self.e_r_of1c = couple(
+            "Accountant", "500", Accounts.CASH, Accounts.RECEIVABLE)
+        self.e_r_of1c.original_entry = self.e_r_or1d
+        self.e_r_of2d, self.e_r_of2c = couple(
+            "Accountant", "200", Accounts.CASH, Accounts.RECEIVABLE)
+        self.e_r_of2c.original_entry = self.e_r_or1d
+        self.e_r_of3d, self.e_r_of3c = couple(
+            "Accountant", "100", Accounts.CASH, Accounts.RECEIVABLE)
+        self.e_r_of3c.original_entry = self.e_r_or1d
+        self.e_r_of4d, self.e_r_of4c = couple(
+            "Toy", "240", Accounts.CASH, Accounts.RECEIVABLE)
+        self.e_r_of4c.original_entry = self.e_r_or2d
+        self.e_r_of5d, self.e_r_of5c = couple(
+            "Interest", "3.4", Accounts.CASH, Accounts.RECEIVABLE)
+        self.e_r_of5c.original_entry = self.e_r_or4d
+
+        # Payable offset entries
+        self.e_p_of1d, self.e_p_of1c = couple(
+            "Airplane ticket", "800", Accounts.PAYABLE, Accounts.CASH)
+        self.e_p_of1d.original_entry = self.e_p_or1c
+        self.e_p_of2d, self.e_p_of2c = couple(
+            "Airplane ticket", "300", Accounts.PAYABLE, Accounts.CASH)
+        self.e_p_of2d.original_entry = self.e_p_or1c
+        self.e_p_of3d, self.e_p_of3c = couple(
+            "Airplane ticket", "100", Accounts.PAYABLE, Accounts.CASH)
+        self.e_p_of3d.original_entry = self.e_p_or1c
+        self.e_p_of4d, self.e_p_of4c = couple(
+            "Phone", "400", Accounts.PAYABLE, Accounts.CASH)
+        self.e_p_of4d.original_entry = self.e_p_or2c
+        self.e_p_of5d, self.e_p_of5c = couple(
+            "Envelop", "0.9", Accounts.PAYABLE, Accounts.CASH)
+        self.e_p_of5d.original_entry = self.e_p_or4c
+
+        # Offset transactions
+        self.t_r_of1: TransactionData = TransactionData(
+            25, [CurrencyData("USD", [self.e_r_of1d], [self.e_r_of1c])])
+        self.t_r_of2: TransactionData = TransactionData(
+            20, [CurrencyData("USD",
+                              [self.e_r_of2d, self.e_r_of3d, self.e_r_of4d],
+                              [self.e_r_of2c, self.e_r_of3c, self.e_r_of4c])])
+        self.t_r_of3: TransactionData = TransactionData(
+            15, [CurrencyData("USD", [self.e_r_of5d], [self.e_r_of5c])])
+        self.t_p_of1: TransactionData = TransactionData(
+            15, [CurrencyData("USD", [self.e_p_of1d], [self.e_p_of1c])])
+        self.t_p_of2: TransactionData = TransactionData(
+            10, [CurrencyData("USD",
+                              [self.e_p_of2d, self.e_p_of3d, self.e_p_of4d],
+                              [self.e_p_of2c, self.e_p_of3c, self.e_p_of4c])])
+        self.t_p_of3: TransactionData = TransactionData(
+            5, [CurrencyData("USD", [self.e_p_of5d], [self.e_p_of5c])])
+
+        self.__add_txn(self.t_r_of1)
+        self.__add_txn(self.t_r_of2)
+        self.__add_txn(self.t_r_of3)
+        self.__add_txn(self.t_p_of1)
+        self.__add_txn(self.t_p_of2)
+        self.__add_txn(self.t_p_of3)
+
+    def __add_txn(self, txn_data: TransactionData) -> None:
+        """Adds a transaction.
+
+        :param txn_data: The transaction data.
+        :return: None.
+        """
+        from accounting.models import Transaction
+        store_uri: str = "/accounting/transactions/store/transfer"
+
+        response: httpx.Response = self.client.post(
+            store_uri, data=txn_data.new_form(self.csrf_token))
+        assert response.status_code == 302
+        txn_id: int = match_txn_detail(response.headers["Location"])
+        txn_data.id = txn_id
+        with self.app.app_context():
+            txn: Transaction | None = db.session.get(Transaction, txn_id)
+            assert txn is not None
+            for i in range(len(txn.currencies)):
+                for j in range(len(txn.currencies[i].debit)):
+                    txn_data.currencies[i].debit[j].id \
+                        = txn.currencies[i].debit[j].id
+                for j in range(len(txn.currencies[i].credit)):
+                    txn_data.currencies[i].credit[j].id \
+                        = txn.currencies[i].credit[j].id

tests/testlib_txn.py

@@ -40,7 +40,10 @@ class Accounts:
     CASH: str = "1111-001"
     PETTY_CASH: str = "1112-001"
     BANK: str = "1113-001"
+    NOTES_RECEIVABLE: str = "1131-001"
+    RECEIVABLE: str = "1141-001"
     PREPAID: str = "1258-001"
+    NOTES_PAYABLE: str = "2131-001"
     PAYABLE: str = "2141-001"
     SALES: str = "4111-001"
     SERVICE: str = "4611-001"
@@ -48,7 +51,7 @@ class Accounts:
     OFFICE: str = "6153-001"
     TRAVEL: str = "6154-001"
     MEAL: str = "6172-001"
-    INTEREST: str = "4111-001"
+    INTEREST: str = "7111-001"
     DONATION: str = "7481-001"
     RENT: str = "7482-001"