foo.diff

     def __eq__(self, other):
-        return isinstance(other, self.__class__) and self._get_pk_val() == other._get_pk_val()
+        if not isinstance(other, Model):
+            return False
+        if self._meta.concrete_model != other._meta.concrete_model:
+            return False
+        my_pk = self._get_pk_val()
+        if my_pk is None:
+            return self is other
+        return my_pk == other._get_pk_val()