Hibernate拆分多对多关系
现在需要将一个多对多的关系拆分，比如学生和老师之间的关系，拆成两个多对一，并且中间表也需要实体类、配置文件。我大概是这样做的：
学生表：
<hibernate-mapping default-lazy="false">
  <class name="Student" table="t_stutent">
  <id name="id" type="long">
  <column name="ID" precision="20" scale="0"/>
  <generator class="sequence">
  <param name="sequence">SEQ_STUTENT</param>
  </generator>
  </id>
  <set name="Teacher" inverse="true" lazy="false" cascade="all-delete-orphan">
  <key column="STUTENT_ID"/>
  <one-to-many class="StudentTeacher"/>
  </set>
  </class>
</hibernate-mapping>

教师表：
<hibernate-mapping default-lazy="false">
  <class name="Teacher" table="t_teacher">
  <id name="id" type="long">
  <column name="ID" precision="20" scale="0"/>
  <generator class="sequence">
  <param name="sequence">SEQ_TEACHER</param>
  </generator>
  </id>
  <set name="stutent" inverse="true" lazy="false" cascade="all-delete-orphan">
  <key column="TEACHER_ID"/>
  <one-to-many class="StutentTeacher"/>
  </set>
  </class>
</hibernate-mapping>

中间表(即StutentTeacher类对应的表)：
<hibernate-mapping default-lazy="false">
  <class name="StutentTeacher" table="t_stu_tea">
  <id name="id" type="long">
  <column name="ID" precision="20" scale="0"/>
  <generator class="sequence">
  <param name="sequence">SEQ_STU_TEA</param>
  </generator>
  </id>
  <property name="studentId" type="long">
  <column name="STUTENT_ID" precision="20"/>
  </property>
  <many-to-one name="stutent" class="Stutent" fetch="select" insert="false" update="false">
  <column name="STUTENT_ID"/>
  </many-to-one>
  <property name="teacherId" type="long">
  <column name="TEACHER_ID" precision="20"/>
  </property>
  <many-to-one name="teacher" class="Teacher" insert="false" update="false" fetch="select">
  <column name="TEACHER_ID"/>
  </many-to-one>
  </class>
</hibernate-mapping>

现在问题是在保存、删除的时候，没有什么问题，中间表中的记录都已经写入了。但是在前端做更新操作时有问题，举例说明，比如stutent1中保存了teacher1，我在前端更新stutent1中为teacher2，这时没问题；但当我更新后希望的是stutent1中保存teacher1和teacher2，这时候就有问题了，它会报：
“org.springframework.orm.hibernate3.HibernateSystemException: A collection with cascade="all-delete-orphan" was no longer reference