说明：

关系操作
1.增加关系相当于在第三张表中插入一行数据
2.解除关系相当于在第三张表中删除一行数据
3.更新关系？
  先删除后增加
4.谁维护关系，效率都一样，看页面的需求

1、多对多的结构是三张表

2、多对多是类与集合的关系

映射文件

在更新课程的时候，级联更新学生

    @Test
    public void testUpdateCourse_Cascade_Update_Student(){
        Session session = sessionFactory.getCurrentSession();
        Transaction transaction =  session.beginTransaction();
        Course course = (Course)session.get(Course.class, 1L);
        //选修该课程的所有的学生
        Set<Student> students = course.getStudents();
        for (Student student : students) {
            student.setName("asfd");
        }
        transaction.commit();
    }
    

    /**
     * 解除课程1和所有的学生之间的关系
     *     delete 
                from
                    student_course 
                where
                    cid=?
               把第三张表中针对cid为1的数据全部删除了

     */
    @Test
    public void testRealseAllR(){
        Session session = sessionFactory.getCurrentSession();
        Transaction transaction =  session.beginTransaction();
        Course course = (Course)session.get(Course.class, 1L);
        course.setStudents(null);
        transaction.commit();
    }
    

    /**
     * 把学生1从课程1转到课程2
     *    Hibernate:   解除关系
            delete 
            from
                student_course 
            where
                sid=? 
                and cid=?
        Hibernate:    增加关系
            insert 
            into
                student_course
                (sid, cid) 
            values
                (?, ?)

     */
    @Test
    public void testStudentFromCourseToCourse(){
        Session session = sessionFactory.getCurrentSession();
        Transaction transaction =  session.beginTransaction();
        Student student = (Student)session.get(Student.class, 1L);
        Course course1 = (Course)session.get(Course.class, 1L);
        Course course2 = (Course)session.get(Course.class, 2L);
        student.getCourses().remove(course1);//解除关系
        student.getCourses().add(course2);//建立关系
        transaction.commit();
    }
    

    /**
     * 删除一个学生
     * Hibernate: 
            delete   解除关系
            from
                student_course 
            where
                sid=?
        Hibernate: 
            delete 删除学生
            from
                Student 
            where
                sid=?
     */
    @Test
    public void testDeleteStudent(){
        Session session = sessionFactory.getCurrentSession();
        Transaction transaction =  session.beginTransaction();
        Student student = (Student)session.get(Student.class, 1L);
        session.delete(student);
        transaction.commit();
    }