code: def fetch_existing_token_of_user(self, client_id, grant_type, user_id):
          
          token_data = self.fetchone(self.fetch_existing_token_of_user_query,
                                     client_id, grant_type, user_id)
  
          if token_data is None:
              raise AccessTokenNotFound
  
          scopes = self._fetch_scopes(access_token_id=token_data[0])
  
          data = self._fetch_data(access_token_id=token_data[0])
  
          return self._row_to_token(data=data, scopes=scopes, row=token_data)
docs: Retrieve an access token issued to a client and user for a specific
          grant.
  
          :param client_id: The identifier of a client as a `str`.
          :param grant_type: The type of grant.
          :param user_id: The identifier of the user the access token has been
                          issued to.
  
          :return: An instance of :class:`oauth2.datatype.AccessToken`.
  
          :raises: :class:`oauth2.error.AccessTokenNotFound` if not access token
                   could be retrieved.