Add protection against mixed up ContentType tables

docs/changelog.rst

@@ -7,6 +7,7 @@ Version 0.5.4 (in development)
 * Fix ``.non_polymorphic()`` to returns a clone of the queryset, instead of effecting the existing queryset.
 * Fix missing ``alters_data = True`` annotations on the overwritten ``save()`` methods.
 * Fix infinite recursion bug in the admin with Django 1.6+
+* Added detection of bad ``ContentType`` table data.
 
 
 Version 0.5.3 (2013-09-17)

polymorphic/polymorphic_model.py

@@ -103,11 +103,19 @@ class PolymorphicModel(six.with_metaclass(PolymorphicModelBase, models.Model)):
         # Note that model_class() can return None for stale content types;
         # when the content type record still exists but no longer refers to an existing model.
         try:
-            return ContentType.objects.get_for_id(self.polymorphic_ctype_id).model_class()
+            model = ContentType.objects.get_for_id(self.polymorphic_ctype_id).model_class()
         except AttributeError:
             # Django <1.6 workaround
             return None
 
+        # Protect against bad imports (dumpdata without --natural) or other
+        # issues missing with the ContentType models.
+        if not issubclass(model, self.__class__):
+            raise RuntimeError("ContentType {0} for {1} #{2} does not point to a subclass!".format(
+                self.polymorphic_ctype_id, model, self.pk,
+            ))
+        return model
+
     def get_real_concrete_instance_class_id(self):
         model_class = self.get_real_instance_class()
         if model_class is None: