Avoid overriding prior calls to select_related in select_subclasses.

CHANGES.rst

@@ -4,6 +4,10 @@ CHANGES
 tip (unreleased)
 ----------------
 
+- Add workaround for https://code.djangoproject.com/ticket/16855 in
+  InheritanceQuerySet to avoid overriding prior calls to
+  ``select_related()``. Thanks ivirabyan.
+
 - Added support for arbitrary levels of model inheritance in
   InheritanceManager. Thanks ivirabyan. (This feature only works in Django
   1.6+ due to https://code.djangoproject.com/ticket/16572).

model_utils/managers.py

@@ -20,7 +20,11 @@ class InheritanceQuerySet(QuerySet):
             if django.VERSION < (1, 6, 0):
                 levels = 1
             subclasses = self._get_subclasses_recurse(self.model, levels=levels)
+        # workaround https://code.djangoproject.com/ticket/16855
+        field_dict = self.query.select_related
         new_qs = self.select_related(*subclasses)
+        if isinstance(new_qs.query.select_related, dict) and isinstance(field_dict, dict):
+            new_qs.query.select_related.update(field_dict)
         new_qs.subclasses = subclasses
         return new_qs
 

model_utils/tests/tests.py

@@ -344,6 +344,17 @@ class InheritanceManagerTests(TestCase):
             self.child1)
 
 
+    def test_prior_select_related(self):
+        # Django 1.2 doesn't have assertNumQueries
+        if django.VERSION >= (1, 3):
+            with self.assertNumQueries(1):
+                obj = self.get_manager().select_related(
+                    "inheritancemanagertestchild1").select_subclasses(
+                    "inheritancemanagertestchild2").get(pk=self.child1.pk)
+                obj.inheritancemanagertestchild1
+
+
+
 class InheritanceManagerRelatedTests(InheritanceManagerTests):
     def setUp(self):
         self.related = InheritanceManagerTestRelated.objects.create()