Set the select clause for the relation query.

        :param columns: The columns
        :type columns: list

        :rtype: list

def _get_select_columns(self, columns=None):
        """
        Set the select clause for the relation query.

        :param columns: The columns
        :type columns: list

        :rtype: list
        """
        if columns == ["*"] or columns is None:
            columns = ["%s.*" % self._related.get_table()]

        return columns + ["%s.%s" % (self._parent.get_table(), self._first_key)]