This method lets you specify the grouping fields explicitly. The `args` must
        be names of grouping fields or calculated fields that this queryset was
        created with.

def group_by(self, *args):
        """
        This method lets you specify the grouping fields explicitly. The `args` must
        be names of grouping fields or calculated fields that this queryset was
        created with.
        """
        for name in args:
            assert name in self._fields or name in self._calculated_fields, \
                   'Cannot group by `%s` since it is not included in the query' % name
        qs = copy(self)
        qs._grouping_fields = args
        return qs