Use empty value Function xxx2Many fields without initial value

Since d142b33fa3ab the on_change_with relies on _changed_value. But for
Function xxx2Many fields if their value is not in the initial value, we should
not retrieve it from the database because the result will be the same as the
one set by on_change_with.

Closes #12836

trytond/trytond/model/modelview.py

@@ -863,6 +863,8 @@ class ModelView(Model):
                 targets = value
                 if fname in init_values:
                     init_targets = init_values._get(fname)
+                elif isinstance(field, fields.Function):
+                    init_targets = []
                 else:
                     init_targets = getattr(init_record, fname, [])
                 value = collections.defaultdict(list)

trytond/trytond/tests/modelview.py

@@ -25,6 +25,12 @@ class ModelViewChangedValues(ModelView):
             ], "MultiSelection")
     dictionary = fields.Dict(
         'test.modelview.changed_values.dictionary', "Dictionary")
+    m2m_function = fields.Function(fields.Many2Many(
+            'test.modelview.changed_values.target', None, None, "Targets"),
+        'on_change_with_m2m_function')
+
+    def on_change_with_m2m_function(self, name=None):
+        return
 
 
 class ModelViewChangedValuesDictSchema(DictSchemaMixin, ModelSQL):

trytond/trytond/tests/test_modelview.py

@@ -109,6 +109,22 @@ class ModelView(unittest.TestCase):
                     },
                 })
 
+    @with_transaction()
+    def test_changed_values_2many_function(self):
+        "Test changed values for xxx2many function field"
+        pool = Pool()
+        Model = pool.get('test.modelview.changed_values')
+
+        with patch.object(
+                Model, 'on_change_with_m2m_function') as on_change_with:
+            on_change_with.return_value = [42]
+            record = Model()
+            record.m2m_function = record.on_change_with_m2m_function()
+
+            self.assertEqual(record._changed_values, {
+                    'm2m_function': {'add': [(0, {'id': 42})]},
+                    })
+
     @with_transaction()
     def test_changed_values_stored(self):
         "Test stored changed values"