Fixed #19195 -- Allow explicit ordering by a relation _id field.

Thanks to chrisedgemon for the report and shaib, akaariai and
timgraham for the review.

tests/ordering/models.py

@@ -17,8 +17,14 @@ from django.db import models
 from django.utils.encoding import python_2_unicode_compatible
 
 
+class Author(models.Model):
+    class Meta:
+        ordering = ('-pk',)
+
+
 @python_2_unicode_compatible
 class Article(models.Model):
+    author = models.ForeignKey(Author, null=True)
     headline = models.CharField(max_length=100)
     pub_date = models.DateTimeField()
 
@@ -27,15 +33,3 @@ class Article(models.Model):
 
     def __str__(self):
         return self.headline
-
-
-@python_2_unicode_compatible
-class ArticlePKOrdering(models.Model):
-    headline = models.CharField(max_length=100)
-    pub_date = models.DateTimeField()
-
-    class Meta:
-        ordering = ('-pk',)
-
-    def __str__(self):
-        return self.headline