JPA系列五：映射关联关系-单向一对多

﹏ヽ暗。殇╰゛Y 2022-02-09 10:49 209阅读 0赞

1、删除实体类Order中的customer属性及其getter和setter方法。  
2、在实体类Customer中添加orders属性及其getter和setter方法。

private Set<Order> orders = new HashSet<>();
    
    //映射单向一对多的关联关系
    //使用 @OneToMany 来映射一对多的关联关系
    //使用 @JoinColumn 来映射外键列的名称
    //可以使用 @OneToMany 的 fetch 属性来修改默认的加载策略
    //可以通过 @OneToMany 的 cascade 属性来修改默认的删除策略.
    @JoinColumn(name="CUSTOMER_ID")
    @OneToMany(fetch=FetchType.LAZY,cascade={CascadeType.REMOVE})
    public Set<Order> getOrders() {
    	return orders;
    }
    
    public void setOrders(Set<Order> orders) {
    	this.orders = orders;
    }

3、测试：

//单向一对多关联关系执行保存时, 一定会多出 UPDATE 语句.
    //因为多的一端在插入时不会同时插入外键列. 
    @Test
    public void testOneToManyPersist(){
    	Customer customer = new Customer();
    	customer.setAge(18);
    	customer.setEmail("mm@163.com");
    	customer.setLastName("MM");
    	Order order1 = new Order();
    	order1.setOrderName("O-MM-1");
    	Order order2 = new Order();
    	order2.setOrderName("O-MM-2");
    	//建立关联关系
    	customer.getOrders().add(order1);
    	customer.getOrders().add(order2);
    	//执行保存操作
    	entityManager.persist(customer);
    	entityManager.persist(order1);
    	entityManager.persist(order2);
    }

控制台输出：

Hibernate: 
        insert 
        into
            JPA_CUTOMERS
            (age, email, LAST_NAME) 
        values
            (?, ?, ?)
    Hibernate: 
        insert 
        into
            JPA_ORDERS
            (ORDER_NAME) 
        values
            (?)
    Hibernate: 
        insert 
        into
            JPA_ORDERS
            (ORDER_NAME) 
        values
            (?)
    Hibernate: 
        update
            JPA_ORDERS 
        set
            CUSTOMER_ID=? 
        where
            id=?
    Hibernate: 
        update
            JPA_ORDERS 
        set
            CUSTOMER_ID=? 
        where
            id=?

//默认对关联的多的一方使用懒加载的加载策略. 
    //可以使用 @OneToMany 的 fetch 属性来修改默认的加载策略
    @Test
    public void testOneToManyFind(){
    	Customer customer = entityManager.find(Customer.class, 9);
    	System.out.println(customer.getLastName());
    	System.out.println(customer.getOrders().size());
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpemhpcWlhbmcxMjE3_size_16_color_FFFFFF_t_70]

//默认情况下, 若删除一的一端, 则会先把关联的多的一端的外键置空, 然后进行删除. 
    //可以通过 @OneToMany 的 cascade 属性来修改默认的删除策略. 
    @Test
    public void testOneToManyRemove(){
    	Customer customer = entityManager.find(Customer.class, 5);
    	entityManager.remove(customer);
    }

Hibernate: 
        select
            customer0_.id as id1_0_0_,
            customer0_.age as age2_0_0_,
            customer0_.email as email3_0_0_,
            customer0_.LAST_NAME as LAST_NAM4_0_0_ 
        from
            JPA_CUTOMERS customer0_ 
        where
            customer0_.id=?
    Hibernate: 
        select
            orders0_.CUSTOMER_ID as CUSTOMER3_0_1_,
            orders0_.id as id1_1_1_,
            orders0_.id as id1_1_0_,
            orders0_.ORDER_NAME as ORDER_NA2_1_0_ 
        from
            JPA_ORDERS orders0_ 
        where
            orders0_.CUSTOMER_ID=?
    Hibernate: 
        update
            JPA_ORDERS 
        set
            CUSTOMER_ID=null 
        where
            CUSTOMER_ID=?
    Hibernate: 
        delete 
        from
            JPA_ORDERS 
        where
            id=?
    Hibernate: 
        delete 
        from
            JPA_ORDERS 
        where
            id=?
    Hibernate: 
        delete 
        from
            JPA_CUTOMERS 
        where
            id=?

@Test
    public void testUpdate(){
    	Customer customer = entityManager.find(Customer.class, 6);
    	customer.getOrders().iterator().next().setOrderName("O-XXX-10");
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpemhpcWlhbmcxMjE3_size_16_color_FFFFFF_t_70 1]

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