[FIX] stock: make it possible to inherit easier in _merge_moves

addons/stock/models/stock_move.py

@@ -514,25 +514,35 @@ class StockMove(models.Model):
             'origin': origin,
         }
 
+    @api.model
+    def _prepare_merge_moves_distinct_fields(self):
+        return [
+            'product_id', 'price_unit', 'product_packaging',
+            'product_uom', 'restrict_partner_id', 'scrapped', 'origin_returned_move_id'
+        ]
+
+    @api.model
+    def _prepare_merge_move_sort_method(self, move):
+        move.ensure_one()
+        return [
+            move.product_id.id, move.price_unit, move.product_packaging.id, move.product_uom.id,
+            move.restrict_partner_id.id, move.scrapped, move.origin_returned_move_id.id
+        ]
+
     def _merge_moves(self):
         """ This method will, for each move in `self`, go up in their linked picking and try to
         find in their existing moves a candidate into which we can merge the move.
         :return: Recordset of moves passed to this method. If some of the passed moves were merged
         into another existing one, return this one and not the (now unlinked) original.
         """
-        distinct_fields = ['product_id', 'price_unit', 'product_packaging',
-                           'product_uom', 'restrict_partner_id', 'scrapped', 'origin_returned_move_id']
-
-        def _keys_sorted(move):
-            return move.product_id.id, move.price_unit, move.product_packaging.id, move.product_uom.id,\
-                   move.restrict_partner_id.id, move.scrapped, move.origin_returned_move_id.id
+        distinct_fields = self._prepare_merge_moves_distinct_fields()
 
         # Move removed after merge
         moves_to_unlink = self.env['stock.move']
         moves_to_merge = []
         for picking in self.mapped('picking_id'):
             # First step find move to merge.
-            for k, g in groupby(sorted(picking.move_lines, key=_keys_sorted), key=itemgetter(*distinct_fields)):
+            for k, g in groupby(sorted(picking.move_lines, key=self._prepare_merge_move_sort_method), key=itemgetter(*distinct_fields)):
                 moves = self.env['stock.move'].concat(*g).filtered(lambda m: m.state not in ('done', 'cancel', 'draft'))
                 # If we have multiple records we will merge then in a single one.
                 if len(moves) > 1: