code: def _get_ref(data, position, obj_end, opts, element_name):
      
      collection, position = _get_string(
          data, position, obj_end, opts, element_name)
      oid, position = _get_oid(data, position, obj_end, opts, element_name)
      return DBRef(collection, oid), position
docs: Decode (deprecated) BSON DBPointer to bson.dbref.DBRef.