jpa级联操作详解5-级联更新2(CascadeType.MERGE)

在上一讲中我们看到两次发出的sql语句是一样的，也就是说两种情况对数据库的操作是一样的，为什么会这样呢？

主要是由于，级联更新在实体是游离状态下才起作用现在依然使用上一讲的实体bean；

第一次在Garage.java 不添加CascadeType.MERGE注解

	@OneToMany(cascade={CascadeType.PERSIST},mappedBy="garage")
	public Set<Auto> getAutos() {
		return autos;
	}

重新编写单元测试方法

	@Test public void update() {
		EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa-hibernate");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Garage garage = em.find(Garage.class, 1);
		em.clear(); //改为游离状态
		garage.setGaragenum("RoomTOP");
		
		em.merge(garage);
		em.getTransaction().commit();
		em.close();
		factory.close();
	}

 运行之后

观察发出的sql语句

Hibernate: select garage0_.gid as gid1_0_, garage0_.garagenum as garagenum1_0_ from Garage garage0_ where garage0_.gid=?
Hibernate: select garage0_.gid as gid1_0_, garage0_.garagenum as garagenum1_0_ from Garage garage0_ where garage0_.gid=?
Hibernate: update Garage set garagenum=? where gid=?

 （二）添加CascadeType.MERGE注解

 

	@OneToMany(cascade={CascadeType.PERSIST,CascadeType.MERGE},mappedBy="garage")
	public Set<Auto> getAutos() {
		return autos;
	}

 运行单元测试方法update()

观察发出的sql语句

Hibernate: select garage0_.gid as gid1_0_, garage0_.garagenum as garagenum1_0_ from Garage garage0_ where garage0_.gid=?
Hibernate: select garage0_.gid as gid1_1_, garage0_.garagenum as garagenum1_1_, autos1_.garageid as garageid3_, autos1_.autoId as autoId3_, autos1_.autoId as autoId0_0_, autos1_.autonum as autonum0_0_, autos1_.autotype as autotype0_0_, autos1_.garageid as garageid0_0_ from Garage garage0_ left outer join Auto autos1_ on garage0_.gid=autos1_.garageid where garage0_.gid=?
Hibernate: update Garage set garagenum=? where gid=?

 比较第一次和第二次发出的sql语句，第二次是做外连接