[ADD] sale_discount_recompute: recompute global discount on order line changes

Ensure that the global discount line in Sale Orders is kept in sync
with the current order lines.

- Recalculate discount when order lines are added, updated, or removed.
- Remove discount line when no order line is present.
- Handle for single discount and multi-discount lines as well.

task-5008128

Author: jcha-odoo

sale_discount_recompute/__init__.py

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+from . import models

sale_discount_recompute/__manifest__.py

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+{
+    'name': 'Sale Discount Recompute',
+    'version': '1.0',
+    'depends': ['sale_management'],
+    'author': 'Jay Chauhan',
+    'category': 'Sales',
+    'summary': 'Automatic Global Discount Recalculation on Sale Orders',
+    'description': '''
+        This feature ensures proper handling of a global discount on Sale Orders.
+
+        - When order lines are added, removed, or updated, the discount line is automatically recalculated to reflect the new subtotal.
+        - If all order lines are removed, the discount line is also removed.
+        - Prevents stale discount amounts from remaining when order content changes.
+    ''',
+    'installable': True,
+    'application': False,
+    'auto_install': False,
+    'license': 'LGPL-3'
+}

sale_discount_recompute/models/__init__.py

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+from . import sale_order

sale_discount_recompute/models/sale_order.py

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+from odoo import Command, api, models
+
+
+class SaleOrder(models.Model):
+    _inherit = "sale.order"
+
+    @api.onchange('order_line')
+    def _onchange_order_line(self):
+        """
+        Triggered in the UI when order lines or global_discount changes.
+        Applies the global discount to each order line.
+        """
+        # Get the default discount productid
+        discount_product_id = self.company_id.sale_discount_product_id
+
+        # Separate Product lines and Discount lines [Old: without changed value, New: with changed value in UI]
+        old_product_lines = self._origin.order_line.filtered(lambda line: line.product_id != discount_product_id)
+        new_product_lines = self.order_line.filtered(lambda line: line.product_id != discount_product_id)
+        old_discount_lines = (self._origin.order_line - old_product_lines)
+        new_discount_lines = (self.order_line - new_product_lines)
+
+        # Calculate Product total
+        old_product_total = sum(old_product_lines.mapped('price_subtotal'))
+        new_product_total = sum(new_product_lines.mapped('price_subtotal'))
+
+        # If product lines exists, calculate new discount unit rates else remove discount lines
+        if new_product_lines:
+            if old_product_total != new_product_total:
+                old_discount_units = old_discount_lines.mapped('price_unit')
+                for index, line in enumerate(new_discount_lines):
+                    discount_percentage = (abs(old_discount_units[index]) / old_product_total * 100) if old_product_total else 0.0
+                    line.price_unit = -((new_product_total * discount_percentage) / 100)
+        else:
+            commands = [Command.unlink(line_id) for line_id in new_discount_lines.mapped('id')]
+            if commands:
+                self.order_line = [c for c in commands]