Fixed #13781 -- Improved select_related in inheritance situations

The select_related code got confused when it needed to travel a
reverse relation to a model which had different parent than the
originally travelled relation.

Thanks to Trac aliases shauncutts for report and ungenio for original
patch (committed patch is somewhat modified version of that).

django/db/models/options.py

@@ -75,6 +75,7 @@ class Options(object):
         from django.db.backends.util import truncate_name
 
         cls._meta = self
+        self.model = cls
         self.installed = re.sub('\.models$', '', cls.__module__) in settings.INSTALLED_APPS
         # First, construct the default values for these options.
         self.object_name = cls.__name__
@@ -464,7 +465,7 @@ class Options(object):
         a granparent or even more distant relation.
         """
         if not self.parents:
-            return
+            return None
         if model in self.parents:
             return [model]
         for parent in self.parents:
@@ -472,8 +473,7 @@ class Options(object):
             if res:
                 res.insert(0, parent)
                 return res
-        raise TypeError('%r is not an ancestor of this model'
-                % model._meta.module_name)
+        return None
 
     def get_parent_list(self):
         """