[FIX] fields: x2many value overrides default value on new records

When instantiating a new record, the commands of an x2many field apply
on an empty recordset instead of the field's default value.  This is
less bug-prone, and avoids an infinite recursion in `convert_to_cache`.

odoo/addons/test_new_api/tests/test_new_fields.py

@@ -958,9 +958,9 @@ class TestFields(common.TransactionCase):
         # no value gives the default value
         new_disc = discussion.new({'name': "Foo"})
         self.assertEqual(new_disc.categories._origin, cat1)
-        # value is combined with default value
+        # value overrides default value
         new_disc = discussion.new({'name': "Foo", 'categories': [(4, cat2.id)]})
-        self.assertEqual(new_disc.categories._origin, cat1 + cat2)
+        self.assertEqual(new_disc.categories._origin, cat2)
 
     def test_40_new_fields(self):
         """ Test new records with relational fields. """

odoo/fields.py

@@ -2321,7 +2321,7 @@ class _RelationalMulti(_Relational):
             else:
                 browse = comodel.browse
             # determine the value ids
-            ids = OrderedSet(record[self.name]._ids)
+            ids = OrderedSet(record[self.name]._ids if validate else ())
             # modify ids with the commands
             for command in value:
                 if isinstance(command, (tuple, list)):