Fixed #17541 -- Fixed non-saved/nullable fk querying

django/db/models/fields/related.py

@@ -496,6 +496,8 @@ class ForeignRelatedObjectsDescriptor(object):
                 except (AttributeError, KeyError):
                     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()
                     qs._known_related_objects = {rel_field: {self.instance.pk: self.instance}}
                     return qs
 

tests/regressiontests/many_to_one_regress/models.py

@@ -48,3 +48,9 @@ class Relation(models.Model):
 
     def __str__(self):
         return "%s - %s" % (self.left.category.name, self.right.category.name)
+
+class Car(models.Model):
+    make = models.CharField(max_length=100, null=True, unique=True)
+
+class Driver(models.Model):
+    car = models.ForeignKey(Car, to_field='make', null=True, related_name='drivers')

tests/regressiontests/many_to_one_regress/tests.py

@@ -4,7 +4,8 @@ from django.db import models
 from django.test import TestCase
 from django.utils import six
 
-from .models import First, Third, Parent, Child, Category, Record, Relation
+from .models import (
+    First, Third, Parent, Child, Category, Record, Relation, Car, Driver)
 
 
 class ManyToOneRegressionTests(TestCase):
@@ -111,3 +112,25 @@ class ManyToOneRegressionTests(TestCase):
         # of a model, and interrogate its related field.
         cat = models.ForeignKey(Category)
         self.assertEqual('id', cat.rel.get_related_field().name)
+
+    def test_relation_unsaved(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")
+        # The object isn't saved an thus the relation field is null - we won't even
+        # execute a query in this case.
+        with self.assertNumQueries(0):
+            self.assertEqual(th.child_set.count(), 0)
+        th.save()
+        # Now the model is saved, so we will need to execute an query.
+        with self.assertNumQueries(1):
+            self.assertEqual(th.child_set.count(), 0)
+    
+    def test_related_null_to_field(self):
+        c1 = Car.objects.create()
+        c2 = Car.objects.create()
+        d1 = Driver.objects.create()
+        self.assertIs(d1.car, None)
+        with self.assertNumQueries(0):
+            self.assertEqual(list(c1.drivers.all()), [])