[1.9.x] Fixed #26092 -- Fixed QuerySet.order_by() regression with an M2M through model.

Backport of ee596888e1 from master

django/db/models/sql/compiler.py

@@ -568,7 +568,7 @@ class SQLCompiler(object):
         # If we get to this point and the field is a relation to another model,
         # append the default ordering for that model unless the attribute name
         # of the field is specified.
-        if field.is_relation and path and opts.ordering and name != field.attname:
+        if field.is_relation and opts.ordering and getattr(field, 'attname', None) != name:
             # Firstly, avoid infinite loops.
             if not already_seen:
                 already_seen = set()

docs/releases/1.9.2.txt

@@ -41,3 +41,6 @@ Bugfixes
 
 * Fixed CSRF cookie check on POST requests when ``USE_X_FORWARDED_PORT=True``
   (:ticket:`26094`).
+
+* Fixed a ``QuerySet.order_by()`` crash when ordering by a relational field of
+  a ``ManyToManyField`` ``through`` model (:ticket:`26092`).

tests/m2m_through/models.py

@@ -65,6 +65,7 @@ class CustomMembership(models.Model):
 
     class Meta:
         db_table = "test_table"
+        ordering = ["date_joined"]
 
 
 class TestNoDefaultsOrNulls(models.Model):

tests/m2m_through/tests.py

@@ -197,6 +197,20 @@ class M2mThroughTests(TestCase):
             attrgetter("name")
         )
 
+    def test_order_by_relational_field_through_model(self):
+        CustomMembership.objects.create(person=self.jim, group=self.rock)
+        CustomMembership.objects.create(person=self.bob, group=self.rock)
+        CustomMembership.objects.create(person=self.jane, group=self.roll)
+        CustomMembership.objects.create(person=self.jim, group=self.roll)
+        self.assertQuerysetEqual(
+            self.rock.custom_members.order_by('custom_person_related_name'),
+            [self.jim, self.bob], lambda x: x
+        )
+        self.assertQuerysetEqual(
+            self.roll.custom_members.order_by('custom_person_related_name'),
+            [self.jane, self.jim], lambda x: x
+        )
+
     def test_query_first_model_by_intermediate_model_attribute(self):
         Membership.objects.create(
             person=self.jane, group=self.roll,