ModelForm now checks whether instance is correct on __init__ (Fixes

django/forms/models.py

@@ -236,6 +236,9 @@ def __init__(self, data=None, files=None, auto_id='id_%s', prefix=None,
             self.instance = opts.model()
             object_data = {}
         else:
+            if not isinstance(instance, opts.model):
+                raise ValueError('"instance" must be an instance of {0}'.format(
+                                    opts.model._meta.object_name))
             self.instance = instance
             object_data = model_to_dict(instance, opts.fields, opts.exclude)
         # if initial was provided, it should override the values from instance

tests/regressiontests/forms/tests/models.py

@@ -114,6 +114,10 @@ def test_unicode_filename(self):
         self.assertEqual(m.file.name, 'tests/\u6211\u96bb\u6c23\u588a\u8239\u88dd\u6eff\u6652\u9c54.txt')
         m.delete()
 
+    def test_incorrect_instance_raises_exception(self):
+        # ModelForm with incorrect instance raises ValueError on init ##########
+        self.assertRaises(ValueError, ChoiceFieldForm, instance=Group)
+
     def test_boundary_conditions(self):
         # Boundary conditions on a PostitiveIntegerField #########################
         class BoundaryForm(ModelForm):