[17.0][FIX] product_secondary_unit: ensure qty persists.

product_secondary_unit/models/product_secondary_unit_mixin.py

@@ -126,6 +126,10 @@ def _compute_helper_target_field_qty(self):
                 rec.secondary_uom_qty * factor,
                 precision_rounding=rec._get_uom_line().rounding,
             )
+            # To avoid resetting the primary quantity to 0.0
+            if not rec.secondary_uom_qty:
+                # Don't modify the primary quantity - preserve it
+                continue
             rec[rec._secondary_unit_fields["qty_field"]] = qty
 
     def _onchange_helper_product_uom_for_secondary(self):

product_secondary_unit/tests/test_secondary_unit_mixin.py

@@ -146,3 +146,21 @@ def test_independent_type(self):
         fake_model.write({"secondary_uom_qty": 4})
         self.assertEqual(fake_model.product_uom_qty, 17)
         self.assertEqual(fake_model.secondary_uom_qty, 4)
+
+    def test_independent_type_with_existing_qty(self):
+        """Test assigning secondary_uom_id to a record with existing qty.
+
+        This should not reset product_uom_qty to 0.0,
+        then switching to independent type, the original qty should be preserved.
+        """
+        fake_model = self.secondary_unit_fake
+        fake_model.product_uom_qty = 10.0
+        fake_model.secondary_uom_id = self.secondary_unit_box_5
+        fake_model.secondary_uom_id.write({"dependency_type": "independent"})
+        previous_product_uom_qty = fake_model.product_uom_qty
+
+        self.assertEqual(previous_product_uom_qty, 10.0)
+        fake_model.write({"secondary_uom_qty": 2})
+
+        self.assertEqual(fake_model.product_uom_qty, previous_product_uom_qty)
+        self.assertEqual(fake_model.secondary_uom_qty, 2)