[FIX] mrp_subcontracting_account: always return PU of subcontracted SM

When using AVCO and subcontracting without any additional cost, the
valuation of the finished product is incorrect

To reproduce the issue:
1. Create a Product Category PC:
    - Costing Method: AVCO
2. Create two products P_compo and P_finished:
    - Both:
        - Type: Storable
        - Category: PC
    - P_compo:
        - Cost: 10
        - Routes: Resupply Subcontractor
3. Update P_compo's quantity: 2
4. Create a Bill of Materials BO:
    - Product: P_finished
    - Type: Subcontracting
    - Subcontractor: a partner P
    - Component: 1 x P_compo
5. In Inventory, create a planned Receipt R:
    - From: P
    - Operations: 1 x P_finished
6. Mark R as To Do
7. Process the delivery of P_compo
8. Validate R
9. Repeat steps 5-8
10. Open the Inventory Valuation

Error: There are two valuation lines for P_finished: one line has a
value of $10, which is correct (this is the cost of the component).
However, the value of the second line is $20, it should be $10 too.

Here are a part of the values used to generate the related MO:
https://github.com/odoo/odoo/blob/2d12fb8fb94c0f2acade7222cfedbec34114a8e9/addons/mrp_subcontracting_account/models/stock_picking.py#L21-L25
In the above case, an extra cost is defined on the MO and is based on
the unit price of the subcontracted SM. However, `_get_price_unit` will
return an incorrect value:
https://github.com/odoo/odoo/blob/251be6b943ea8c3f274bb0863d0af3f7c6b8d10d/addons/stock_account/models/stock_move.py#L39
After step 8, the standard price of P_finished is $10. Also, in the
above case, there isn't any subcontracting cost, so there isn't any unit
price defined on the SM. Therefore, `_get_price_unit` returns the
standard price of P_finished ($10). As a result, when computing the
value of the finished stock move:
https://github.com/odoo/odoo/blob/4fc2ec31861d4602357f130066ec3507b96d8dc8/addons/mrp_account/models/mrp_production.py#L39-L42
It uses the extra cost of the MO + the component cost. This explains
where the $20 come from.

OPW-2641339

closes odoo/odoo#80889

X-original-commit: 0567536f8b66c71f6860041926542f88026793b1
Signed-off-by: William Henrotin (whe) <whe@odoo.com>
Signed-off-by: Adrien Widart <awt@odoo.com>

addons/mrp_subcontracting_account/models/__init__.py

@@ -3,3 +3,4 @@
 
 from . import stock_picking
 from . import product_product
+from . import stock_move

addons/mrp_subcontracting_account/models/stock_move.py

@@ -0,0 +1,12 @@
+# -*- coding: utf-8 -*-
+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+
+from odoo import models
+
+
+class StockMove(models.Model):
+    _inherit = 'stock.move'
+
+    def _should_force_price_unit(self):
+        self.ensure_one()
+        return self.is_subcontract or super()._should_force_price_unit()

addons/mrp_subcontracting_account/tests/test_subcontracting_account.py

@@ -68,6 +68,26 @@ class TestAccountSubcontractingFlows(TestMrpSubcontractingCommon):
         self.assertEqual(picking_receipt.move_ids.stock_valuation_layer_ids.value, 0)
         self.assertEqual(picking_receipt.move_ids.product_id.value_svl, 60)
 
+        # Do the same without any additionnal cost
+        picking_form = Form(self.env['stock.picking'])
+        picking_form.picking_type_id = self.env.ref('stock.picking_type_in')
+        picking_form.partner_id = self.subcontractor_partner1
+        with picking_form.move_ids_without_package.new() as move:
+            move.product_id = self.finished
+            move.product_uom_qty = 1
+        picking_receipt = picking_form.save()
+        picking_receipt.move_ids.price_unit = 0
+
+        picking_receipt.action_confirm()
+        picking_receipt.move_ids.quantity_done = 1.0
+        picking_receipt._action_done()
+
+        mo = picking_receipt._get_subcontract_production()
+        # In this case, since there isn't any additionnal cost, the total cost of the subcontracting
+        # is the sum of the components' costs: 10 + 20 = 30
+        self.assertEqual(mo.move_finished_ids.stock_valuation_layer_ids.value, 30)
+        self.assertEqual(picking_receipt.move_ids.product_id.value_svl, 90)
+
     def test_subcontracting_account_backorder(self):
         """ This test uses tracked (serial and lot) component and tracked (serial) finished product
         The original subcontracting production order will be split into 4 backorders. This test

addons/stock_account/models/stock_move.py

@@ -34,6 +34,10 @@ class StockMove(models.Model):
         self.analytic_account_line_id.unlink()
         return super()._action_cancel()
 
+    def _should_force_price_unit(self):
+        self.ensure_one()
+        return False
+
     def _get_price_unit(self):
         """ Returns the unit price to value this stock move """
         self.ensure_one()
@@ -42,7 +46,7 @@ class StockMove(models.Model):
         # If the move is a return, use the original move's price unit.
         if self.origin_returned_move_id and self.origin_returned_move_id.sudo().stock_valuation_layer_ids:
             price_unit = self.origin_returned_move_id.sudo().stock_valuation_layer_ids[-1].unit_cost
-        return not float_is_zero(price_unit, precision) and price_unit or self.product_id.standard_price
+        return price_unit if not float_is_zero(price_unit, precision) or self._should_force_price_unit() else self.product_id.standard_price
 
     @api.model
     def _get_valued_types(self):