Hibernate 关系映射 many-to-many

╰半夏微凉° 2024-02-17 18:17 37阅读 0赞

**1 创建数据表**

drop table studentm;
    
    create table studentm
    (
     sid number(5) primary key,
     sname varchar2(20) not null
    
    )
    
    drop table teacherm;
    
    create table teacherm
    (
     tid number(5) primary key,
     tname varchar2(20) not null
    
    )
    
    drop table teacherm_studentm;
    
    --组合PK  (不需要创建关系)
    create table teacherm_studentm
    (
     msid number(5) not null,
     mtid number(5)  not null,
      primary key(msid,mtid)
    )
    
    select * from teacherm;
    select * from studentm;
    select * from teacherm_studentm
    
    create sequence seq_ssid
    start with 1
    cache 20;
    
    create sequence seq_tid
    start with 1000
    cache 20;

2.创建实体类及映射文件

Studentm.java

//学生有多个老师  getXXX and setXXx
    	private Set<Teacherm> teacherms = new HashSet<Teacherm>();
    
    	// Constructors
    
    	public Set<Teacherm> getTeacherms() {
    		return teacherms;
    	}
    
    	public void setTeacherms(Set<Teacherm> teacherms) {
    		this.teacherms = teacherms;
    	}

Studentm.hbm.xml

Teacherm.java

//老师有多个学生  getXXX and setXXx
    	private Set<Studentm>  studentms = new HashSet<Studentm>();
    
    	// Constructors
    
    	public Set<Studentm> getStudentms() {
    		return studentms;
    	}
    
    	public void setStudentms(Set<Studentm> studentms) {
    		this.studentms = studentms;
    	}

Teacherm.hbm.xml

3.测试添加

@Test
    	public void save(){
    		
    		//1)获得会话
    		Session session = HibernateUtil.currentSession();
    		
    		//事务对象
    		Transaction tx = session.beginTransaction();
    		
    		//学生对象
    		Studentm stu1 = new Studentm("张三");
    		Studentm stu2 = new Studentm("沈彦");
    		Studentm stu3 = new Studentm("王五");
    		Studentm stu4= new Studentm("刘飞");
    		
    		//老师对象
    		Teacherm t1 = new Teacherm("祝一帆");
    		Teacherm t2 = new Teacherm("许路阳");
    		
    		
    		//学生包含老师
    		stu1.getTeacherms().add(t1);
    		stu2.getTeacherms().add(t1);
    		stu3.getTeacherms().add(t2);
    		stu4.getTeacherms().add(t2);
    		
    		
    		//老师包含学生
    		t1.getStudentms().add(stu1);
    		t1.getStudentms().add(stu2);
    		t2.getStudentms().add(stu3);
    		t2.getStudentms().add(stu4);
    		
    		
    		//保存老师
    		session.save(t1);
    		session.save(t2);
    		
    		
    		//提交事务
    		tx.commit();
    		
    		//2)关闭会话
    		HibernateUtil.closeSession();
    		
    	}

对应的SQL语句：

![Center][]

4.测试删除

@Test
    	public void del(){
    		
    		//1)获得会话
    		Session session = HibernateUtil.currentSession();
    		
    		//事务对象
    		Transaction tx = session.beginTransaction();
    		  
    		//加载老师1000
    		Teacherm t1 =(Teacherm) session.load(Teacherm.class, 1002);
    		
    		session.delete(t1);
    		
    		//提交事务
    		tx.commit();
    		
    		//2)关闭会话
    		HibernateUtil.closeSession();
    		
    	}

对应的SQL语句：  
![Center 1][]

总之：

如果inverse=false,也就是自己控制，为主控方！

两个实体类只进行了老师维护关联关系cascade=all，而学生将控制权交给老师这方来管理inverse=true;

再将Studentm实体对象添加到Teacherm的studentms属性集合中时才能更新维护中间表。也就是只要添加老师，或删除老师就OK了！

[Center]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/79395e3a3a2b44a68bfcfd79d18109b5.png
[Center 1]: https://image.dandelioncloud.cn/pgy_files/images/2024/01/29/29651aee593949acb940457324283e29.png