Fixed #24201 -- Added order_with_respect_to support to GenericForeignKey.

tests/order_with_respect_to/models.py

@@ -1,5 +1,9 @@
 """
 Tests for the order_with_respect_to Meta attribute.
+
+We explicitly declare app_label on these models, because they are reused by
+contenttypes_tests. When those tests are run in isolation, these models need
+app_label because order_with_respect_to isn't in INSTALLED_APPS.
 """
 
 from django.db import models
@@ -10,6 +14,9 @@ from django.utils.encoding import python_2_unicode_compatible
 class Question(models.Model):
     text = models.CharField(max_length=200)
 
+    class Meta:
+        app_label = 'order_with_respect_to'
+
 
 @python_2_unicode_compatible
 class Answer(models.Model):
@@ -18,6 +25,7 @@ class Answer(models.Model):
 
     class Meta:
         order_with_respect_to = 'question'
+        app_label = 'order_with_respect_to'
 
     def __str__(self):
         return six.text_type(self.text)
@@ -30,6 +38,7 @@ class Post(models.Model):
 
     class Meta:
         order_with_respect_to = "parent"
+        app_label = 'order_with_respect_to'
 
     def __str__(self):
         return self.title