Ticket #17541: 17541-2.diff

django/db/models/fields/related.py

@@ -458 +458 @@
                     return self.instance._prefetched_objects_cache[rel_field.related_query_name()]
                 except (AttributeError, KeyError):
                     db = self._db or router.db_for_read(self.model, instance=self.instance)
+                    if getattr(self.instance, attname) is None:
+                        return super(RelatedManager, self).get_query_set().using(db).none()
                     return super(RelatedManager, self).get_query_set().using(db).filter(**self.core_filters)
 
             def get_prefetch_query_set(self, instances):

tests/regressiontests/many_to_one_regress/tests.py

@@ -106 +106 @@
         # of a model, and interrogate its related field.
         cat = models.ForeignKey(Category)
         self.assertEqual('id', cat.rel.get_related_field().name)
+
+    def test_related_relation_on_none(self):
+        # Test that the <field>_set manager does not join on Null value fields (#17541)
+        Third.objects.create(name='Third 1')
+        Third.objects.create(name='Third 2')
+        th = Third(name="testing")
+        self.assertEqual(th.child_set.count(), 0)