              source.save()
  
      def test_foreign_key_retrieve_with_null(self):
          queryset = NullableForeignKeySource.objects.all()
          serializer = NullableSlugSourceSerializer(queryset)
          expected = [
              {'id': 1, 'name': u'source-1', 'target': 'target-1'},
              {'id': 2, 'name': u'source-2', 'target': 'target-1'},
              {'id': 3, 'name': u'source-3', 'target': None},
          ]
 
</s> remove         serializer = NullableSlugSourceSerializer(queryset)
 </s> add         serializer = NullableForeignKeySourceSerializer(queryset) </s> remove         serializer = NullableSlugSourceSerializer(queryset)
 </s> add         serializer = NullableForeignKeySourceSerializer(queryset) </s> remove         serializer = NullableSlugSourceSerializer(queryset)
 </s> add         serializer = NullableForeignKeySourceSerializer(queryset) </s> remove         serializer = NullableSlugSourceSerializer(queryset)
 </s> add         serializer = NullableForeignKeySourceSerializer(queryset)
        serializer = NullableForeignKeySourceSerializer(queryset)
              source.save()
  
      def test_foreign_key_retrieve_with_null(self):
          queryset = NullableForeignKeySource.objects.all()
        serializer = NullableForeignKeySourceSerializer(queryset)
          expected = [
              {'id': 1, 'name': u'source-1', 'target': 'target-1'},
              {'id': 2, 'name': u'source-2', 'target': 'target-1'},
              {'id': 3, 'name': u'source-3', 'target': None},
          ]
keep keep keep keep replace keep keep keep keep keep
 Raise Validation Errors when relationships receive incorrect types. Fixes #590. 
rest_framework/tests/relations_slug.py