docs: def _as_document(self, identifier):
          """ Converts given identifier to the document indexed by FTS backend.
  
          Args:
              identifier (dict): identifier to convert. Dict contains at
                  least 'identifier', 'type' and 'name' keys.
  
          Returns:
              dict with structure matches to BaseIdentifierIndex._schema.
  
          """
          return {
              'identifier': u('{}').format(identifier['identifier']),
              'type': u('{}').format(identifier['type']),
              'name': u('{}').format(identifier['name'])
          }