[1.6.x] Fixed #21568 -- Added missing ModelMultipleChoiceField to_python method

Thanks dibrovsd at gmail.com for the report and Simon Charette
for the review.
Backport of 8a9c8bb90 from master.

django/forms/models.py

@@ -1167,6 +1167,12 @@ class ModelMultipleChoiceField(ModelChoiceField):
             msg = _('Hold down "Control", or "Command" on a Mac, to select more than one.')
             self.help_text = string_concat(self.help_text, ' ', msg)
 
+    def to_python(self, value):
+        if not value:
+            return []
+        to_py = super(ModelMultipleChoiceField, self).to_python
+        return [to_py(val) for val in value]
+
     def clean(self, value):
         if self.required and not value:
             raise ValidationError(self.error_messages['required'], code='required')

docs/releases/1.6.1.txt

@@ -22,6 +22,9 @@ Bug fixes
   raised an error (#21439).
 * Fixed a regression that prevented editable ``GenericRelation`` subclasses
   from working in ``ModelForms``.
+* Added missing ``to_python`` method for ``ModelMultipleChoiceField`` which
+  is required in Django 1.6 to properly detect changes from initial values
+  (#21568).
 * Fixed ``django.contrib.humanize`` translations where the unicode sequence
   for the non-breaking space was returned verbatim (#21415).
 * Fixed :djadmin:`loaddata` error when fixture file name contained any dots

tests/model_forms_regress/tests.py

@@ -44,6 +44,29 @@ class ModelMultipleChoiceFieldTests(TestCase):
         f.clean([p.pk for p in Person.objects.all()[8:9]])
         self.assertTrue(self._validator_run)
 
+    def test_model_multiple_choice_show_hidden_initial(self):
+        """
+        Test support of show_hidden_initial by ModelMultipleChoiceField.
+        """
+        class PersonForm(forms.Form):
+            persons = forms.ModelMultipleChoiceField(show_hidden_initial=True,
+                                                     queryset=Person.objects.all())
+
+        person1 = Person.objects.create(name="Person 1")
+        person2 = Person.objects.create(name="Person 2")
+
+        form = PersonForm(initial={'persons': [person1, person2]},
+                          data={'initial-persons': [str(person1.pk), str(person2.pk)],
+                                'persons': [str(person1.pk), str(person2.pk)]})
+        self.assertTrue(form.is_valid())
+        self.assertFalse(form.has_changed())
+
+        form = PersonForm(initial={'persons': [person1, person2]},
+                          data={'initial-persons': [str(person1.pk), str(person2.pk)],
+                                'persons': [str(person2.pk)]})
+        self.assertTrue(form.is_valid())
+        self.assertTrue(form.has_changed())
+
 
 class TripleForm(forms.ModelForm):
     class Meta: