[1.5.x] Fixed #19652 -- Fixed .none() regression in related fields

The regression was caused by using .none() when querying for related
models, and the origin field's value was None. This resulted in missing
custom related manager subclass as .none() returns plain QuerySet.

This isn't backport from master, in master .none() correctly preserves
the queryset's class.

Patch provided by Simon Charette, with some minor polish by committer.

django/db/models/fields/related.py

@@ -498,7 +498,8 @@ class ForeignRelatedObjectsDescriptor(object):
                     db = self._db or router.db_for_read(self.model, instance=self.instance)
                     qs = super(RelatedManager, self).get_query_set().using(db).filter(**self.core_filters)
                     if getattr(self.instance, attname) is None:
-                        return qs.none()
+                        # We don't want to use qs.none() here, see #19652
+                        return qs.filter(pk__in=[])
                     qs._known_related_objects = {rel_field: {self.instance.pk: self.instance}}
                     return qs
 

tests/modeltests/custom_managers/models.py

@@ -63,3 +63,24 @@ class Car(models.Model):
 
     def __str__(self):
         return self.name
+
+
+# Bug #19652
+class ObjectQuerySet(models.query.QuerySet):
+    pass
+
+class ObjectManager(models.Manager):
+    use_for_related_fields = True
+
+    def get_query_set(self):
+        return ObjectQuerySet(self.model, using=self._db)
+
+
+class RelatedObject(models.Model):
+    pass
+
+
+class Object(models.Model):
+    related = models.ForeignKey(RelatedObject, related_name='objs')
+
+    objects = ObjectManager()

tests/modeltests/custom_managers/tests.py

@@ -3,7 +3,8 @@ from __future__ import absolute_import
 from django.test import TestCase
 from django.utils import six
 
-from .models import Person, Book, Car, PersonManager, PublishedBookManager
+from .models import (ObjectQuerySet, RelatedObject, Person, Book, Car, PersonManager,
+    PublishedBookManager)
 
 
 class CustomManagerTests(TestCase):
@@ -72,3 +73,13 @@ class CustomManagerTests(TestCase):
             ],
             lambda c: c.name
         )
+
+    def test_related_manager(self):
+        """
+        Make sure un-saved object's related managers always return an instance
+        of the same class the manager's `get_query_set` returns. Refs #19652.
+        """
+        rel_qs = RelatedObject().objs.all()
+        self.assertIsInstance(rel_qs, ObjectQuerySet)
+        with self.assertNumQueries(0):
+            self.assertFalse(rel_qs.exists())