jpa 中的CascadeType.Merge

关于配置一对多的多端的cascadetype的问题。

//学生类
public class Student{ 
    private String s_id;
    private ClassRoom classRoom;

//....getter and setter      
@ManyToOne(fetch = FetchType.LAZY,cascade = CascadeType.MERGE)      
@JoinColumn(name="c_id")     
public Class getClassRoom() { return classRoom;}
}

//教室类
public class ClassRoom{
    private String c_id;
    pricate Set<Student> students = new HashSet<Studnet>(0);

//...getter and setter
} 

//测试类
public class Test{
    public void test(){
        //新建一个教室对象
        ClassRoom cr = new ClassRomm();
        cr.setC_id("1");

        //新建一个学生对象并且关联教室
        Student stu = new Student();
        stu.setS_id("201501");
        stu.setClassRoom(cr);

        //调用JPA的persistent方法
        entityManager.persistent(stu);
    }
}


 //异常信息
Caused by: org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: ClassRoom -> ClassRoom
    at org.hibernate.engine.CascadingAction$9.noCascade(CascadingAction.java:387)
    at org.hibernate.engine.Cascade.cascade(Cascade.java:172)
    at org.hibernate.event.def.AbstractFlushingEventListener.cascadeOnFlush(AbstractFlushingEventListener.java:154)
    at org.hibernate.event.def.AbstractFlushingEventListener.prepareEntityFlushes(AbstractFlushingEventListener.java:145)
    at org.hibernate.event.def.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:88)
    at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:50)
    at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1216)
    at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:383)
    at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:133)
    at org.hibernate.ejb.TransactionImpl.commit(TransactionImpl.java:76)
    ... 36 more

我想为什么要在保存Student之前要先persistentClassRoom，我不是设置了CascadeType.MERGE了么。。。。原以为这里的MERGE和Hibernate的saveOrUpdate是一个意思，难道不是麼，，对于ORM框架本人是初学者，，烦请指教，谢谢

（为了突出问题，删掉了不必要的代码，见谅）