returns an SqlaXlsExporter for the given related object and stores it in
        the column object as a cache

def _get_related_exporter(self, related_obj, column):
        """
        returns an SqlaXlsExporter for the given related object and stores it in
        the column object as a cache
        """
        result = column.get('sqla_xls_exporter')
        if result is None:
            worksheet = self.book.create_sheet(
                title=column.get('label', 'default title')
            )
            result = column['sqla_xls_exporter'] = SqlaXlsExporter(
                related_obj.__class__,
                worksheet=worksheet
            )
        return result