[18.0][FIX] account_payment_multi_deduction: preserve multi deductions when changing the payment journal

account_payment_multi_deduction/models/__init__.py

@@ -1,3 +1,4 @@
 # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html)
 
 from . import account_move
+from . import account_payment

account_payment_multi_deduction/models/account_payment.py

@@ -0,0 +1,32 @@
+# Copyright 2026 Ecosoft Co., Ltd (http://ecosoft.co.th/)
+# License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html)
+
+from odoo import models
+
+
+class AccountPayment(models.Model):
+    _inherit = "account.payment"
+
+    def _prepare_move_lines_per_type(
+        self, write_off_line_vals=None, force_balance=None
+    ):
+        """Re-expand multi deduction write-off lines collapsed by core resync."""
+        if write_off_line_vals and len(write_off_line_vals) == 1:
+            deduct_lines = self.move_id.line_ids.filtered("is_writeoff")
+            if len(deduct_lines) > 1:
+                write_off_line_vals = [
+                    {
+                        "name": line.name,
+                        "account_id": line.account_id.id,
+                        "partner_id": line.partner_id.id,
+                        "currency_id": line.currency_id.id,
+                        "amount_currency": line.amount_currency,
+                        "balance": line.balance,
+                        "analytic_distribution": line.analytic_distribution,
+                        "is_writeoff": True,
+                    }
+                    for line in deduct_lines
+                ]
+        return super()._prepare_move_lines_per_type(
+            write_off_line_vals=write_off_line_vals, force_balance=force_balance
+        )

account_payment_multi_deduction/tests/test_payment_multi_deduction.py

@@ -185,3 +185,61 @@ def test_04_one_invoice_payment_with_keep_open(self):
         writeoff = payment.move_id.line_ids.filtered(lambda line: line.is_writeoff)
         self.assertEqual(len(writeoff), 1)
         self.assertEqual(writeoff.account_id, self.account_expense)
+
+    def test_05_reset_and_change_journal_keeps_writeoff_lines(self):
+        """Multi deduction write-off lines must survive a reset + journal change"""
+        ctx = {
+            "active_ids": [self.cust_invoice.id],
+            "active_id": self.cust_invoice.id,
+            "active_model": "account.move",
+        }
+        with Form(
+            self.payment_register_model.with_context(**ctx), view=self.register_view_id
+        ) as f:
+            f.amount = 400.0  # Reduce to 400.0, and mark fully paid (multi)
+            f.payment_difference_handling = "reconcile_multi_deduct"
+            with f.deduction_ids.new() as f2:
+                f2.account_id = self.account_expense
+                f2.name = "Expense 1"
+                f2.amount = 20.0
+            with f.deduction_ids.new() as f2:
+                f2.account_id = self.account_expense
+                f2.name = "Expense 2"
+                f2.amount = 30.0
+        payment_register = f.save()
+        payment = payment_register._create_payments()
+
+        # Sanity check: 2 write-off lines created.
+        writeoff = payment.move_id.line_ids.filtered(lambda line: line.is_writeoff)
+        self.assertEqual(len(writeoff), 2)
+        original_total = sum(writeoff.mapped("balance"))
+
+        # Reset the payment to draft and switch its journal.
+        payment.action_draft()
+        # Setup a second bank journal with a valid outstanding account, so the
+        # resynchronization triggered by the journal change can rebuild the
+        # liquidity lines.
+        other_journal = self.journal_model.create(
+            {
+                "name": "Bank Other",
+                "type": "bank",
+                "code": "BNK2",
+                "company_id": self.company_data["company"].id,
+            }
+        )
+        outstanding = self.env["account.chart.template"].ref(
+            "account_journal_payment_debit_account_id"
+        )
+        other_journal.inbound_payment_method_line_ids[
+            :1
+        ].payment_account_id = outstanding
+        other_journal.outbound_payment_method_line_ids[
+            :1
+        ].payment_account_id = outstanding
+        payment.journal_id = other_journal
+
+        # The write-off lines must still be there, line by line.
+        writeoff = payment.move_id.line_ids.filtered(lambda line: line.is_writeoff)
+        self.assertEqual(len(writeoff), 2)
+        self.assertEqual(writeoff.mapped("account_id"), self.account_expense)
+        self.assertEqual(sum(writeoff.mapped("balance")), original_total)