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.

tests/one_to_one/tests.py

@@ -373,13 +373,15 @@ 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)