Provide a better error message when polymorphic_ctype_id is Null

refs #51, #140, #304

(cherry picked from commit fb8eed78ad)

polymorphic/models.py

@@ -17,6 +17,10 @@ from .query_translate import translate_polymorphic_Q_object
 # PolymorphicModel
 
 
+class PolymorphicTypeUndefined(LookupError):
+    pass
+
+
 class PolymorphicModel(six.with_metaclass(PolymorphicModelBase, models.Model)):
     """
     Abstract base class that provides polymorphic behaviour
@@ -82,6 +86,13 @@ class PolymorphicModel(six.with_metaclass(PolymorphicModelBase, models.Model)):
         retrieve objects, then the real class/type of these objects may be
         determined using this method.
         """
+        if self.polymorphic_ctype_id is None:
+            raise PolymorphicTypeUndefined((
+                "The model {}#{} does not have a `polymorphic_ctype_id` value defined.\n"
+                "If you created models outside polymorphic, e.g. through an import or migration, "
+                "make sure the `polymorphic_ctype_id` field points to the ContentType ID of the model subclass."
+            ).format(self.__class__.__name__, self.pk))
+
         # the following line would be the easiest way to do this, but it produces sql queries
         # return self.polymorphic_ctype.model_class()
         # so we use the following version, which uses the ContentType manager cache.

polymorphic/tests/test_orm.py

@@ -5,6 +5,7 @@ from django.db.models import Case, Count, Q, When
 from django.test import TestCase
 from django.utils import six
 from polymorphic.contrib.guardian import get_polymorphic_base_content_type
+from polymorphic.models import PolymorphicTypeUndefined
 from polymorphic.tests import *  # all models
 
 
@@ -783,6 +784,16 @@ class PolymorphicTests(TestCase):
         ctype = get_polymorphic_base_content_type(Model2D)
         self.assertEqual(ctype.name, 'model2a')
 
+    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):
     """