在上一讲中jpa级联操作详解2 Garage.java中有一个CascadeType.REMOVE注解，是在删除garage表中数据的时候级联删除auto表中的数据；这次我们研究在Auto中有一个CascadeType.REMOVE注解时，在删除auto表中的数据的时候能否级联删除garage表中的数据 （一）不在Auto.java添加CascadeType.REMOVE注解时 数据库中的数据如下 Sql代码 收藏代码
mysql> select * from auto; +--------+---------+----------+----------+ | autoId | autonum | autotype | garageid | +--------+---------+----------+----------+ | 1 | hk2222 | car | 1 | | 2 | bj0000 | car | 1 | +--------+---------+----------+----------+C#代码 收藏代码
mysql> select * from garage; +-----+-----------+ | gid | garagenum | +-----+-----------+ | 1 | room1 | +-----+-----------+运行单元测试方法 delete2() Java代码 收藏代码
@Test public void delete2() { EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa-hibernate"); EntityManager em = factory.createEntityManager(); em.getTransaction().begin(); Auto auto4 = em.find(Auto.class, 1); Auto auto5 = em.find(Auto.class, 2); em.remove(auto4); em.remove(auto5); em.getTransaction().commit(); em.close(); factory.close(); }单元测试成功，auto中对应的两条字段被删除 发出的sql语句为： Sql代码 收藏代码
Hibernate: select auto0_.autoId as autoId0_1_, auto0_.autonum as autonum0_1_, auto0_.autotype as autotype0_1_, auto0_.garageid as garageid0_1_, garage1_.gid as gid1_0_, garage1_.garagenum as garagenum1_0_ from Auto auto0_ left outer join Garage garage1_ on auto0_.garageid=garage1_.gid where auto0_.autoId=? Hibernate: select auto0_.autoId as autoId0_1_, auto0_.autonum as autonum0_1_, auto0_.autotype as autotype0_1_, auto0_.garageid as garageid0_1_, garage1_.gid as gid1_0_, garage1_.garagenum as garagenum1_0_ from Auto auto0_ left outer join Garage garage1_ on auto0_.garageid=garage1_.gid where auto0_.autoId=? Hibernate: delete from Auto where autoId=? Hibernate: delete from Auto where autoId=?虽然auto中对应的garage字段被删除，但是garage字段gid=1时虽然在auto表中没有了对应的记录，但是这条数据依然存在
mysql> select * from garage; +-----+-----------+ | gid | garagenum | +-----+-----------+ | 1 | room1 | +-----+-----------+ -----------------------------------------（二）在Auto.java中加入CascadeType.REMOVE字段 Java代码 收藏代码
@ManyToOne(cascade={CascadeType.REMOVE}) @JoinColumn(name="garageid") public Garage getGarage() { return garage; }复原数据库中的数据为 Sql代码 收藏代码
mysql> select * from auto; +--------+---------+----------+----------+ | autoId | autonum | autotype | garageid | +--------+---------+----------+----------+ | 1 | hk2222 | car | 1 | | 2 | bj0000 | car | 1 | +--------+---------+----------+----------+C#代码 收藏代码
mysql> select * from garage; +-----+-----------+ | gid | garagenum | +-----+-----------+ | 1 | room1 | +-----+-----------+重新运行delete2()方法，测试成功 打开数据库，数据显示auto表和garage表中的相关数据都被删除了 Sql代码 收藏代码
mysql> select * from auto; Empty set (0.00 sec)Sql代码 收藏代码
mysql> select * from garage; Empty set (0.00 sec)回看一下这次发出的sql语句为 Sql代码 收藏代码
Hibernate: select auto0_.autoId as autoId0_1_, auto0_.autonum as autonum0_1_, auto0_.autotype as autotype0_1_, auto0_.garageid as garageid0_1_, garage1_.gid as gid1_0_, garage1_.garagenum as garagenum1_0_ from Auto auto0_ left outer join Garage garage1_ on auto0_.garageid=garage1_.gid where auto0_.autoId=? Hibernate: select auto0_.autoId as autoId0_1_, auto0_.autonum as autonum0_1_, auto0_.autotype as autotype0_1_, auto0_.garageid as garageid0_1_, garage1_.gid as gid1_0_, garage1_.garagenum as garagenum1_0_ from Auto auto0_ left outer join Garage garage1_ on auto0_.garageid=garage1_.gid where auto0_.autoId=? Hibernate: update Auto set autonum=?, autotype=?, garageid=? where autoId=? Hibernate: delete from Auto where autoId=? Hibernate: delete from Garage where gid=? Hibernate: delete from Auto where autoId=?注意 ：如果delete2方法中改为：只删除一条记录，这样就会出错 Sql代码 收藏代码
@Test public void delete2() { EntityManagerFactory factory = Persistence.createEntityManagerFactory("jpa-hibernate"); EntityManager em = factory.createEntityManager(); em.getTransaction().begin(); Auto auto4 = em.find(Auto.class, 1); em.remove(auto4); em.getTransaction().commit(); em.close(); factory.close(); }观察发出的sql语句 Sql代码 收藏代码
Hibernate: select auto0_.autoId as autoId0_1_, auto0_.autonum as autonum0_1_, auto0_.autotype as autotype0_1_, auto0_.garageid as garageid0_1_, garage1_.gid as gid1_0_, garage1_.garagenum as garagenum1_0_ from Auto auto0_ left outer join Garage garage1_ on auto0_.garageid=garage1_.gid where auto0_.autoId=? Hibernate: delete from Auto where autoId=? Hibernate: delete from Garage where gid=?我可以发现由于数据auto中有两条记录，先输出了id为1的字段记录后，由于又要删除garage中的相应记录 Sql代码 收藏代码
delete from Garage where gid=?但是由于还有一个外键关联相应的garage gid=1的记录，所以删除会报错
