Fixed problem with refs #10811.

When 'to_field' is specified with a FK, then we need to check the pk value
the object.

django/db/models/fields/related.py

@@ -623,13 +623,13 @@ class ReverseSingleRelatedObjectDescriptor(object):
         # Set the values of the related field.
         else:
             for lh_field, rh_field in self.field.related_fields:
-                val = getattr(value, rh_field.attname)
-                if val is None:
+                pk = value._get_pk_val()
+                if pk is None:
                     raise ValueError(
                         'Cannot assign "%r": "%s" instance isn\'t saved in the database.' %
                         (value, self.field.rel.to._meta.object_name)
                     )
-                setattr(instance, lh_field.attname, val)
+                setattr(instance, lh_field.attname, getattr(value, rh_field.attname))
 
         # Since we already know what the related object is, seed the related
         # object caches now, too. This avoids another db hit if you get the

tests/many_to_one_regress/models.py

@@ -26,7 +26,7 @@ class Third(models.Model):
 
 
 class Parent(models.Model):
-    name = models.CharField(max_length=20)
+    name = models.CharField(max_length=20, unique=True)
     bestchild = models.ForeignKey('Child', null=True, related_name='favored_by')
 
 
@@ -35,6 +35,10 @@ class Child(models.Model):
     parent = models.ForeignKey(Parent)
 
 
+class ToFieldChild(models.Model):
+    parent = models.ForeignKey(Parent, to_field='name')
+
+
 # Multiple paths to the same model (#7110, #7125)
 @python_2_unicode_compatible
 class Category(models.Model):

tests/many_to_one_regress/tests.py

@@ -4,7 +4,7 @@ from django.db import models
 from django.test import TestCase
 
 from .models import (
-    First, Third, Parent, Child, Category, Record, Relation, Car, Driver)
+    First, Third, Parent, Child, ToFieldChild, Category, Record, Relation, Car, Driver)
 
 
 class ManyToOneRegressionTests(TestCase):
@@ -71,6 +71,11 @@ class ManyToOneRegressionTests(TestCase):
                 % (p, Child.parent.field.rel.to._meta.object_name)):
             Child(parent=p)
 
+        with self.assertRaisesMessage(ValueError,
+                'Cannot assign "%r": "%s" instance isn\'t saved in the database.'
+                % (p, Child.parent.field.rel.to._meta.object_name)):
+            ToFieldChild(parent=p)
+
         # Creation using attname keyword argument and an id will cause the
         # related object to be fetched.
         p = Parent.objects.get(name="Parent")