[1.8.x] Fixed #25160 (again) -- Moved data loss check on reverse relations.

Moved data loss check when assigning to a reverse one-to-one relation on
an unsaved instance to Model.save(). This is exactly the same change as
e4b813c but for reverse relations.

Backport of c3904de from master

django/db/models/fields/related.py

@@ -506,12 +506,6 @@ class SingleRelatedObjectDescriptor(object):
                     raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)
 
         related_pk = tuple(getattr(instance, field.attname) for field in self.related.field.foreign_related_fields)
-        if None in related_pk:
-            raise ValueError(
-                'Cannot assign "%r": "%s" instance isn\'t saved in the database.' %
-                (value, instance._meta.object_name)
-            )
-
         # Set the value of the related field to the value of the related object's related field
         for index, field in enumerate(self.related.field.local_related_fields):
             setattr(value, field.attname, related_pk[index])

docs/releases/1.8.5.txt

@@ -39,3 +39,7 @@ Bugfixes
 
 * Fixed custom queryset chaining with ``values()`` and ``values_list()``
   (:ticket:`20625`).
+
+* Moved the :ref:`unsaved model instance assignment data loss check
+  <unsaved-model-instance-check-18>` on reverse relations to ``Model.save()``
+  (:ticket:`25160`).

docs/releases/1.8.txt

@@ -701,6 +701,9 @@ Assigning unsaved objects to relations raises an error
         ...
         ValueError: save() prohibited to prevent data loss due to unsaved related object 'author'.
 
+    A similar check on assignment to reverse one-to-one relations was removed
+    in Django 1.8.5.
+
 Assigning unsaved objects to a :class:`~django.db.models.ForeignKey`,
 :class:`~django.contrib.contenttypes.fields.GenericForeignKey`, and
 :class:`~django.db.models.OneToOneField` now raises a :exc:`ValueError`.

tests/one_to_one/tests.py

@@ -371,13 +371,15 @@ class OneToOneTests(TestCase):
         """
         p = Place()
         b = UndergroundBar.objects.create()
-        msg = (
-            'Cannot assign "<UndergroundBar: UndergroundBar object>": "Place" '
-            'instance isn\'t saved in the database.'
-        )
+
+        # Assigning a reverse relation on an unsaved object is allowed.
+        p.undergroundbar = b
+
+        # However saving the object is not allowed.
+        msg = "save() prohibited to prevent data loss due to unsaved related object 'place'."
         with self.assertNumQueries(0):
             with self.assertRaisesMessage(ValueError, msg):
-                p.undergroundbar = b
+                b.save()
 
     def test_nullable_o2o_delete(self):
         u = UndergroundBar.objects.create(place=self.p1)