def only(self, *fields):
        '''
        Restricts the fields to be fetched when mapping. Set to `__model` to fetch all fields define in the ModelIndex.
        '''
        s = self._clone()
        if len(fields) == 1 and fields[0] == '__model':
            s._only = '__model'
        else:
            s._only = fields
        return s
Restricts the fields to be fetched when mapping. Set to `__model` to fetch all fields define in the ModelIndex.