          serializer.save()
  
          # Ensure source 1 is updated, and everything else is as expected
          queryset = NullableForeignKeySource.objects.all()
          serializer = NullableSlugSourceSerializer(queryset)
          expected = [
              {'id': 1, 'name': u'source-1', 'target': None},
              {'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) </s> remove         serializer = NullableSlugSourceSerializer(instance, data=data)
 </s> add         serializer = NullableForeignKeySourceSerializer(instance, data=data)
        serializer = NullableForeignKeySourceSerializer(queryset)
          serializer.save()
  
          # Ensure source 1 is updated, and everything else is as expected
          queryset = NullableForeignKeySource.objects.all()
        serializer = NullableForeignKeySourceSerializer(queryset)
          expected = [
              {'id': 1, 'name': u'source-1', 'target': None},
              {'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