[IMP] sale: auto-recalculate global discount on order line changes

Enhanced the Sales Order logic to automatically recalculate and update the
global discount line whenever sale order lines are added, updated, or deleted.
This ensures that the discount remains in sync with the current order total.
If all product lines are removed, the discount line is also removed.
This improves data consistency and reduces the need for manual discount updates.
task-4962386

Author: niyp-odoo

discount_update/__init__.py

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+from . import models

discount_update/__manifest__.py

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+{
+    'name': "Discount Update",
+    'version': "1.0",
+    'depends': ["sale_management"],
+    'insallable': True,
+    'license': 'LGPL-3'
+}

discount_update/models/__init__.py

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+from . import sale_order
+from . import sale_order_line

discount_update/models/sale_order.py

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+from odoo import fields, models
+
+
+class SaleOrder(models.Model):
+    _inherit = "sale.order"
+
+    wizard_ids = fields.One2many(
+        "sale.order.discount", "sale_order_id", string="Discount Wizards"
+    )
+
+    def _update_discount(self):
+        discount_product = self.env["product.product"].search([("name", "=", "Discount")], limit=1)
+        if not discount_product:
+            return
+
+        total_amount = sum(line.price_subtotal for line in self.order_line if not line.is_discount_line())
+
+        discount_lines = self.order_line.filtered(lambda l: l.is_discount_line())
+
+        if not total_amount:
+            discount_lines.unlink()
+            return
+
+        if self.wizard_ids:
+            discount_percentage = self.wizard_ids[-1].discount_percentage
+        else:
+            discount_lines.unlink()
+            return
+
+        discount_amount = total_amount * discount_percentage
+
+        if len(discount_lines) > 1:
+            discount_lines[1:].unlink()
+            discount_line = discount_lines[0]
+        elif discount_lines:
+            discount_line = discount_lines[0]
+        else:
+            discount_line = False
+
+        vals = {
+            "product_id": discount_product.id,
+            "price_unit": -discount_amount,
+            "name": f"Discount {discount_percentage * 100:.2f}%",
+            "tax_id": [(6, 0, [])],
+        }
+
+        if discount_line:
+            discount_line.write(vals)
+        else:
+            self.order_line.create({
+                **vals,
+                "order_id": self.id,
+                "product_uom_qty": 1,
+            })

discount_update/models/sale_order_line.py

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+from odoo import api, models
+
+
+class SaleOrderLine(models.Model):
+    _inherit = "sale.order.line"
+
+    def is_discount_line(self):
+        discount_product = self.order_id.env["product.product"].search([("name", "=", "Discount")], limit=1)
+        return self.product_id == discount_product
+
+    def unlink(self):
+        order_ids = self.mapped("order_id")
+        res = super().unlink()
+        for order in order_ids:
+            order._update_discount()
+        return res
+
+    @api.model_create_multi
+    def create(self, vals):
+        lines = super().create(vals)
+        for line in lines:
+            if line.order_id and not line.is_discount_line():
+                line.order_id._update_discount()
+        return lines
+
+    def write(self, vals):
+        res = super().write(vals)
+        for line in self:
+            if not line.is_discount_line():
+                line.order_id._update_discount()
+        return res