Fixed test failure caused by different NULL ordering between backends

django/db/backends/__init__.py

@@ -599,6 +599,9 @@ class BaseDatabaseFeatures(object):
     # to remove any ordering?
     requires_explicit_null_ordering_when_grouping = False
 
+    # Does the backend order NULL values as largest or smallest?
+    nulls_order_largest = False
+
     # Is there a 1000 item limit on query parameters?
     supports_1000_query_parameters = True
 

django/db/backends/postgresql_psycopg2/base.py

@@ -58,6 +58,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     can_distinct_on_fields = True
     can_rollback_ddl = True
     supports_combined_alters = True
+    nulls_order_largest = True
 
 
 class DatabaseWrapper(BaseDatabaseWrapper):

tests/queries/models.py

@@ -262,9 +262,13 @@ class ReservedName(models.Model):
         return self.name
 
 # A simpler shared-foreign-key setup that can expose some problems.
+@python_2_unicode_compatible
 class SharedConnection(models.Model):
     data = models.CharField(max_length=10)
 
+    def __str__(self):
+        return self.data
+
 class PointerA(models.Model):
     connection = models.ForeignKey(SharedConnection)
 

tests/queries/tests.py

@@ -2984,7 +2984,11 @@ class Ticket14056Tests(TestCase):
         s2 = SharedConnection.objects.create(data='s2')
         s3 = SharedConnection.objects.create(data='s3')
         PointerA.objects.create(connection=s2)
+        expected_ordering = (
+            [s1, s3, s2] if connection.features.nulls_order_largest
+            else [s2, s1, s3]
+        )
         self.assertQuerysetEqual(
-            SharedConnection.objects.order_by('pointera__connection', 'pk'),
-            [s1, s3, s2], lambda x: x
+            SharedConnection.objects.order_by('-pointera__connection', 'pk'),
+            expected_ordering, lambda x: x
         )