Fixed #20836 -- Ensure that the ForeignKey's to_field attribute is properly considered by the admin's interface when creating related objects.

Many thanks to Collin Anderson for the report and patch and to Peter Sheats for the test.

tests/admin_widgets/models.py

@@ -130,3 +130,11 @@ class School(models.Model):
 
     def __str__(self):
         return self.name
+
+
+@python_2_unicode_compatible
+class Profile(models.Model):
+    user = models.ForeignKey('auth.User', 'username')
+
+    def __str__(self):
+        return self.user.username