Allow 'None' to pass as a null value in RelatedFields

rest_framework/relations.py

@@ -33,6 +33,7 @@ class RelatedField(WritableField):
     many_widget = widgets.SelectMultiple
     form_field_class = forms.ChoiceField
     many_form_field_class = forms.MultipleChoiceField
+    null_values = (None, '', 'None')
 
     cache_choices = False
     empty_label = None
@@ -168,9 +169,9 @@ class RelatedField(WritableField):
                 return
             value = [] if self.many else None
 
-        if value in (None, '') and self.required:
+        if value in self.null_values:
+            if self.required:
                 raise ValidationError(self.error_messages['required'])
-        elif value in (None, ''):
             into[(self.source or field_name)] = None
         elif self.many:
             into[(self.source or field_name)] = [self.from_native(item) for item in value]

rest_framework/tests/test_nullable_fields.py

@@ -15,12 +15,12 @@ urlpatterns = patterns(
 
 class NullableForeignKeyTests(APITestCase):
     """
-    DRF should be able to handle nullable fields when a TestClient
-    POST/PUT request is made with its own serialized object.
+    DRF should be able to handle nullable foreign keys when a test
+    Client POST/PUT request is made with its own serialized object.
     """
     urls = 'rest_framework.tests.test_nullable_fields'
 
-    def test_updating_object_with_null_field_value(self):
+    def test_updating_object_with_null_fk(self):
         obj = NullableForeignKeySource(name='example', target=None)
         obj.save()
         serialized_data = NullableFKSourceSerializer(obj).data