Hibernate set many-to-many EnumType 枚举类型的set集合怎么多对多关联起来？

Question

枚举类
public enum PurchaseAttachmentType {
REQUIREMENTS,
SIGN,
OPINION,
REPROT,
ANNOUNCEMENT,
AGENCY,

TRANSACTION
;
}

public class ProcurementMethod {

private Set purchaseAttachmentTypes;
}

hibernate配置文件（省略了部分）

文件类型


下面多对多的关系该怎么写？