Provide a better error message when polymorphic_ctype_id is Null

refs #51, #140, #304

polymorphic/tests/test_orm.py

@@ -4,6 +4,8 @@ import django
 from django.db.models import Case, Count, Q, When
 from django.test import TestCase
 from django.utils import six
+
+from polymorphic.models import PolymorphicTypeUndefined
 from polymorphic.tests import *  # all models
 
 
@@ -765,6 +767,16 @@ class PolymorphicTests(TestCase):
         result = Model2B.objects.annotate(val=Concat('field1', 'field2'))
         self.assertEqual(list(result), [])
 
+    def test_null_polymorphic_id(self):
+        """Test that a proper error message is displayed when the database lacks the ``polymorphic_ctype_id``"""
+        Model2A.objects.create(field1='A1')
+        Model2B.objects.create(field1='A1', field2='B2')
+        Model2B.objects.create(field1='A1', field2='B2')
+        Model2A.objects.all().update(polymorphic_ctype_id=None)
+
+        with self.assertRaises(PolymorphicTypeUndefined):
+            list(Model2A.objects.all())
+
 
 def qrepr(data):
     """