[1.8.x] Fixed #24578 -- Fixed crash with QuerySet.update() on FK to O2O fields.

Thanks Anssi Kääriäinen for review.

Backport of 10b4c010ab2cdaa6ba8bfaec3e3540299ea77be from master

django/db/models/base.py

@@ -912,7 +912,7 @@ class Model(six.with_metaclass(ModelBase)):
     def prepare_database_save(self, field):
         if self.pk is None:
             raise ValueError("Unsaved model instance %r cannot be used in an ORM query." % self)
-        return getattr(self, field.rel.field_name)
+        return getattr(self, field.rel.get_related_field().attname)
 
     def clean(self):
         """

docs/releases/1.8.1.txt

@@ -28,3 +28,6 @@ Bugfixes
 
 * Fixed a migration crash when altering
   :class:`~django.db.models.ManyToManyField`\s (:ticket:`24513`).
+
+* Fixed a crash with ``QuerySet.update()`` on foreign keys to one-to-one fields
+  (:ticket:`24578`).

tests/one_to_one/tests.py

@@ -468,3 +468,14 @@ class OneToOneTests(TestCase):
         # refs #21563
         self.assertFalse(hasattr(Director(), 'director'))
         self.assertFalse(hasattr(School(), 'school'))
+
+    def test_update_one_to_one_pk(self):
+        p1 = Place.objects.create()
+        p2 = Place.objects.create()
+        r1 = Restaurant.objects.create(place=p1)
+        r2 = Restaurant.objects.create(place=p2)
+        w = Waiter.objects.create(restaurant=r1)
+
+        Waiter.objects.update(restaurant=r2)
+        w.refresh_from_db()
+        self.assertEqual(w.restaurant, r2)