diff --git a/django/db/models/base.py b/django/db/models/base.py
index f2404d40a5c..2b79a20cee2 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -826,10 +826,10 @@ class Model(six.with_metaclass(ModelBase)):
             setattr(self, cachename, obj)
         return getattr(self, cachename)
 
-    def prepare_database_save(self, unused):
+    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 self.pk
+        return getattr(self, field.rel.field_name)
 
     def clean(self):
         """
diff --git a/tests/update/models.py b/tests/update/models.py
index ebc9c3353c9..e0b391d0cdf 100644
--- a/tests/update/models.py
+++ b/tests/update/models.py
@@ -42,3 +42,11 @@ class C(models.Model):
 
 class D(C):
     a = models.ForeignKey(A)
+
+
+class Foo(models.Model):
+    target = models.CharField(max_length=10, unique=True)
+
+
+class Bar(models.Model):
+    foo = models.ForeignKey(Foo, to_field='target')
diff --git a/tests/update/tests.py b/tests/update/tests.py
index 95e9069972f..578b3ab4402 100644
--- a/tests/update/tests.py
+++ b/tests/update/tests.py
@@ -2,7 +2,7 @@ from __future__ import unicode_literals
 
 from django.test import TestCase
 
-from .models import A, B, D, DataPoint, RelatedPoint
+from .models import A, B, D, DataPoint, RelatedPoint, Foo, Bar
 
 
 class SimpleTest(TestCase):
@@ -125,3 +125,16 @@ class AdvancedTests(TestCase):
         method = DataPoint.objects.all()[:2].update
         self.assertRaises(AssertionError, method,
                           another_value='another thing')
+
+    def test_update_respects_to_field(self):
+        """
+        Update of an FK field which specifies a to_field works.
+        """
+        a_foo = Foo.objects.create(target='aaa')
+        b_foo = Foo.objects.create(target='bbb')
+        bar = Bar.objects.create(foo=a_foo)
+        self.assertEqual(bar.foo_id, a_foo.target)
+        bar_qs = Bar.objects.filter(pk=bar.pk)
+        self.assertEqual(bar_qs[0].foo_id, a_foo.target)
+        bar_qs.update(foo=b_foo)
+        self.assertEqual(bar_qs[0].foo_id, b_foo.target)