docs: def dereference(self, session, ref, allow_none=False):
          """ Dereference a pymongo "DBRef" to this field's underlying type """
          from ommongo.document import collection_registry
          # TODO: namespace support
          ref.type = collection_registry['global'][ref.collection]
          obj = session.dereference(ref, allow_none=allow_none)
          return obj