Set the fields returned the query.

        :type projection: str or sequence of strings
        :param projection: Each value is a string giving the name of a
                           property to be included in the projection query.

def projection(self, projection):
        """Set the fields returned the query.

        :type projection: str or sequence of strings
        :param projection: Each value is a string giving the name of a
                           property to be included in the projection query.
        """
        if isinstance(projection, str):
            projection = [projection]
        self._projection[:] = projection