def METHOD_NAME(self) -> str:
    with db.session.no_autoflush:
        identifier = self.issued_at.strftime('%Y%mU-') + '%06d' % (
            EventInvoice.query.count() + 1
        )
        count = EventInvoice.query.filter_by(identifier=identifier).count()
        if count == 0:
            return identifier
        return self.METHOD_NAME()