def keys(self, **kwargs):
        """
        DEPRECATED
        Iterator that returns primary keys as a sequence of dicts.
        """
        warnings.warn('Use of `rel.fetch.keys()` notation is deprecated. '
                      'Please use `rel.fetch("KEY")` or `rel.fetch(dj.key)` for equivalent result', stacklevel=2)
        yield from self._expression.proj().fetch(as_dict=True, **kwargs)
DEPRECATED
        Iterator that returns primary keys as a sequence of dicts.