fix-#397: Allow specifying the '_cls' as a field for indexes

mongoengine/dereference.py

@@ -180,7 +180,11 @@ class DeReference(object):
                 return self.object_map.get(items['_ref'].id, items)
             elif '_cls' in items:
                 doc = get_document(items['_cls'])._from_son(items)
+                _cls = doc._data.pop('_cls', None)
+                del items['_cls']
                 doc._data = self._attach_objects(doc._data, depth, doc, None)
+                if _cls is not None:
+                    doc._data['_cls'] = _cls
                 return doc
 
         if not hasattr(items, 'items'):