[ADD] discount_update: update global discount on order line changes

- Implemented dynamic recalculation of global discount lines whenever a
product is added, removed, or updated in the sale order lines.
- Prevents outdated or incorrect discount amounts by cleaning and
 recreating discount lines on each change.
- Handles both single-tax and multi-tax scenarios to preserve tax consistency
 in discounts.

Author: mamg-odoo

discount_update/__init__.py

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+from . import models

discount_update/__manifest__.py

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+{
+    "name": "Update Discount OL",
+    "depends": ["base", "sale", "sale_management"],
+    "data": [],
+    "license": "LGPL-3",
+}

discount_update/models/__init__.py

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+from . import sale_order_inherit
+from . import sale_order_line_inherit

discount_update/models/sale_order_inherit.py

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+from collections import defaultdict
+from odoo import _, models
+
+
+class SaleOrderInherit(models.Model):
+    _inherit = "sale.order"
+
+    def recalculate_discount(self):
+        for order in self:
+            discount_ol = order.order_line.filtered(
+                lambda ol: ol.is_global_discount_line
+            )
+            if not discount_ol:
+                return
+
+            discount_per = discount_ol[0].discount_percentage
+            discount_product = discount_ol[0].product_id
+            discount_ol.unlink()
+            total_price_per_tax_groups = defaultdict(float)
+
+            for line in order.order_line:
+                if not line.product_uom_qty or not line.price_unit:
+                    continue
+
+                total_price_per_tax_groups[line.tax_id] += (
+                    line.price_unit * line.product_uom_qty
+                )
+
+            if not total_price_per_tax_groups:
+                return
+
+            elif len(total_price_per_tax_groups) == 1:
+                taxes = next(iter(total_price_per_tax_groups.keys()))
+                subtotal = total_price_per_tax_groups[taxes]
+                self.env["sale.order.line"].create(
+                    {
+                        "order_id": order.id,
+                        "product_id": discount_product.id,
+                        "name": _("Discount: %(percent)s%%", percent=discount_per),
+                        "product_uom_qty": 1,
+                        "tax_id": [(6, 0, taxes.ids)],
+                        "price_unit": -subtotal * discount_per / 100,
+                    }
+                )
+
+            else:
+                vals_list = [
+                    (
+                        {
+                            "order_id": order.id,
+                            "product_id": discount_product.id,
+                            "name": _(
+                                "Discount: %(percent)s%%"
+                                "- On products with the following taxes %(taxes)s",
+                                percent=discount_per,
+                                taxes=", ".join(taxes.mapped("name")),
+                            ),
+                            "product_uom_qty": 1,
+                            "tax_id": [(6, 0, taxes.ids)],
+                            "price_unit": -subtotal * discount_per / 100,
+                        }
+                    )
+                    for taxes, subtotal in total_price_per_tax_groups.items()
+                ]
+                self.env["sale.order.line"].create(vals_list)

discount_update/models/sale_order_line_inherit.py

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+import re
+from odoo import api, fields, models
+
+
+class SaleOrderLineInherit(models.Model):
+    _inherit = "sale.order.line"
+
+    is_global_discount_line = fields.Boolean(default=False)
+    discount_percentage = fields.Float()
+
+    @api.model_create_multi
+    def create(self, vals_list):
+        res = super().create(vals_list)
+        for line in res:
+            if "%" in line.name:
+                match = re.search(r"(\d+(?:\.\d+)?)%", line.name)
+                if match:
+                    value = float(match.group(1))
+                    line.discount_percentage = value
+                line.is_global_discount_line = True
+            else:
+                line.order_id.recalculate_discount()
+        return res
+
+    def write(self, vals):
+        if self.env.context.get("skip_recalculate_discount"):
+            return super().write(vals)
+
+        res = super().write(vals)
+
+        for line in self:
+            line.order_id.with_context(
+                skip_recalculate_discount=True
+            ).recalculate_discount()
+        return res
+
+    def unlink(self):
+        orders = self.filtered(lambda ol: not ol.is_global_discount_line).mapped(
+            "order_id"
+        )
+        res = super().unlink()
+        for order in orders:
+            order.recalculate_discount()
+        return res