子类之间many-to-many关联出现问题

super class 为 Subject，两个子类User和UserGroup

做多对多关联无法在关联表建立外键约束
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
??? "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
??? "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="uac.model.businessobjects">
?<class name="Subject" table="tbSubject_info" abstract="true">
??<id name="sequenceId" type="integer" column="PK_Sequence">
???<generator type="string"
???length="1">
??</discriminator>
??<!--维护主体与角色的多对多关联 -->
??
??<property name="subjectId" type="string" column="LK_SubjectID"
???length="10" unique="true">
??</property>
??<subclass name="User" discriminator-value="U" extends="Subject">

???<!--维护用户与用户组的多对多关联 -->
???<set name="groups" inverse="true" table="tbUser_Org_Rel">
????<key column="FK_UserID" property-ref="userId" ></key>
????<many-to-many column="FK_OrgID"
?????property-ref="orgId" />
???</set>

???<join table="tbUser_info" >
????
????<key column="PK_USER" foreign-key="User_Subject_FK">
????</key>

????<property name="userId" column="VC_UserID" type="string"
?????length="10" not-null="true" unique="true">
????</property>
???????</join>

??</subclass>
??<subclass name="UserGroup" discriminator-value="G" extends="Subject">

???<!--维护用户组与用户的多对多关联 -->
???<set name="users" inverse="true" table="tbUser_Org_Rel">
????<key column="FK_OrgID" property-ref="orgId"></key>
????<many-to-many column="FK_UserID" property-ref="userId" />
???</set>

???<join table="tbUserGroup_info">
????<key column="PK_UserGroup"
?????foreign-key="UserGroup_Subject_FK">
????</key>
????<property name="orgId" column="VC_OrgID" type="string"
?????length="10" not-null="true" unique="true">
????</property>
???
???</join>

??</subclass>

?</class>

</hibernate-mapping>