code: def one(self):
          
          first = self.first()
          second = next(self, None)
          if second is not None:
              raise ConstraintViolation("{} found more than one result.".format(self.mode.capitalize()))
          return first
docs: Return the unique result.  If there is not exactly one result,
          raises :exc:`~bloop.exceptions.ConstraintViolation`.
  
          :return: The unique result.
          :raises bloop.exceptions.ConstraintViolation: Not exactly one result.