[FIX] product_multi_company_stock: Adding a company to a product must not affect existing lots

Bug: adding a new company to a product's company_ids triggers a
recompute of stock.lot.company_id (which depends on product_id.company_id).
Both lots evaluate product_id.company_id in the same env context, so
they end up with the same company_id, causing _check_unique_lot to raise
a ValidationError even though no lot data changed.

Author: LoisRForgeFlow

product_multi_company_stock/models/__init__.py

@@ -1 +1,2 @@
 from . import product_template
+from . import stock_lot

product_multi_company_stock/models/stock_lot.py

@@ -0,0 +1,23 @@
+# Copyright 2026 ForgeFlow S.L. (http://www.forgeflow.com)
+# License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
+
+from odoo import api, models
+
+
+class StockLot(models.Model):
+    _inherit = "stock.lot"
+
+    @api.depends("product_id.company_id", "product_id.company_ids")
+    def _compute_company_id(self):
+        # For lots belonging to single-company products, defer to standard logic.
+        # For multi-company products, preserve the stored company_id so that a
+        # change to product.company_ids (e.g. adding a third company) never
+        # overwrites the lot's original company — which would cause _check_unique_lot
+        # to fire when two lots share the same name across different companies.
+        single_company_lots = self.filtered(
+            lambda rec: len(rec.product_id.sudo().company_ids) == 1
+        )
+        res = super(StockLot, single_company_lots)._compute_company_id()
+        for lot in self - single_company_lots:
+            lot.company_id = lot._origin.company_id or lot.product_id.company_id
+        return res

product_multi_company_stock/tests/test_product_multi_company_stock.py

@@ -1,5 +1,4 @@
-# Copyright 2025 ForgeFlow S.L.
-#   (http://www.forgeflow.com)
+# Copyright 2025-26 ForgeFlow S.L. (http://www.forgeflow.com)
 # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
 
 from odoo.exceptions import UserError
@@ -116,3 +115,47 @@ def test_remove_company_with_quants_or_moves_archived_product(self):
         with self.assertRaises(UserError) as error_move:
             product.write({"company_ids": [(6, 0, [self.company_2.id])]})
         self.assertIn("stock moves", str(error_move.exception))
+
+    def test_add_company_does_not_break_same_name_lots(self):
+        """Adding a company to a product must not affect existing lots.
+
+        Adding a new company to a product's company_ids triggers a
+        recompute of stock.lot.company_id (which depends on product_id.company_id).
+        should not change lots company.
+        """
+        product = self.env["product.product"].create(
+            {
+                "name": "Test Tracked Product",
+                "is_storable": True,
+                "tracking": "lot",
+                "company_ids": [(6, 0, [self.company_1.id, self.company_2.id])],
+            }
+        )
+        company_3 = self.company_obj.create({"name": "Test company 3"})
+        lot_1 = (
+            self.env["stock.lot"]
+            .with_company(self.company_1)
+            .create(
+                {
+                    "name": "LOT001",
+                    "product_id": product.id,
+                    "company_id": self.company_1.id,
+                }
+            )
+        )
+        lot_2 = (
+            self.env["stock.lot"]
+            .with_company(self.company_2)
+            .create(
+                {
+                    "name": "LOT001",
+                    "product_id": product.id,
+                    "company_id": self.company_2.id,
+                }
+            )
+        )
+        # Adding a third company should neither raise nor alter the lot companies.
+        # This currently raises a ValidationError from _check_unique_lot — the bug.
+        product.write({"company_ids": [(4, company_3.id)]})
+        self.assertEqual(lot_1.company_id, self.company_1)
+        self.assertEqual(lot_2.company_id, self.company_2)