Fixed a bug with Django >= 1.9 where values_list was called on

InheritanceQuerySet with `select_subclasses` applied as strings
raised AttributeError exception.

Adds a new test case `test_dj19_values_list_on_select_subclasses`

AUTHORS.rst

@@ -2,6 +2,7 @@ ad-m <github.com/ad-m>
 Alejandro Varas <alej0varas@gmail.com>
 Alex Orange <crazycasta@gmail.com>
 Andy Freeland <andy@andyfreeland.net>
+Artis Avotins <artis.avotins@gmail.com>
 Bram Boogaard <b.boogaard@auto-interactive.nl>
 Carl Meyer <carl@dirtcircle.com>
 Curtis Maloney <curtis@tinbrain.net>

CHANGES.rst

@@ -4,6 +4,9 @@ CHANGES
 master (unreleased)
 -------------------
 
+* Fix `InheritanceQuerySet` raising an `AttributeError` exception
+  under Django 1.9.
+
 
 2.5 (2016-04-18)
 ----------------

model_utils/managers.py

@@ -161,6 +161,12 @@ class InheritanceQuerySetMixin(object):
 
     def _get_sub_obj_recurse(self, obj, s):
         rel, _, s = s.partition(LOOKUP_SEP)
+
+        # Django 1.9: If a primitive type gets passed to this recursive function,
+        # return None as non-models are not part of inheritance.
+        if not isinstance(obj, models.Model):
+            return None
+
         try:
             node = getattr(obj, rel)
         except ObjectDoesNotExist:

model_utils/tests/tests.py

@@ -768,6 +768,54 @@ class InheritanceManagerTests(TestCase):
         self.assertEqual(list(queryset), [{'id': self.child1.pk}])
 
 
+    @skipUnless(django.VERSION >= (1, 9, 0), "test only applies to Django 1.9+")
+    def test_dj19_values_list_on_select_subclasses(self):
+        """
+        Using `select_subclasses` in conjunction with `values_list()` raised an
+        exception in `_get_sub_obj_recurse()` because the result of `values_list()`
+        is either a `tuple` or primitive objects if `flat=True` is specified,
+        because no type checking was done prior to fetching child nodes.
+
+        Django versions below 1.9 are not affected by this bug.
+        """
+
+        # Querysets are cast to lists to force immediate evaluation.
+        # No exceptions must be thrown.
+
+        # No argument to select_subclasses
+        objs_1 = list(
+            self.get_manager().
+            select_subclasses().
+            values_list('id')
+        )
+
+        # String argument to select_subclasses
+        objs_2 = list(
+            self.get_manager().
+            select_subclasses(
+                "inheritancemanagertestchild2"
+            ).
+            values_list('id')
+        )
+
+        # String argument to select_subclasses
+        objs_3 = list(
+            self.get_manager().
+            select_subclasses(
+                InheritanceManagerTestChild2
+            ).
+            values_list('id')
+        )
+
+        assert all((
+            isinstance(objs_1, list),
+            isinstance(objs_2, list),
+            isinstance(objs_3, list),
+        ))
+
+        assert objs_1 == objs_2 == objs_3
+
+
 class InheritanceManagerUsingModelsTests(TestCase):
 
     def setUp(self):