Hibernate JPA Training-4

接着之前的blog继续介绍hibernate one-to-many关系，将涉及到orphanRemoval使用，one-to-many单向和双向关联不同产生的问题。

1- 准备工作

2- One-To-Many 介绍

## 2.1- One-To-Many 级联更新子表(使用mappedBy,实体关系的owner为character) —

执行如下代码

@Test
@Rollback(false)
public void testSaveAccount(){
    entityManager.persist(account);
    for(Character character: account.getCharacters()){
        if(character.getCharacterId()==1){
            account.getCharacters().remove(character);
        }
    }
    entityManager.merge(account);
}

我们可以看到log删除了子表的一条character数据

2013-03-29 11:52:59,331 DEBUG [org.hibernate.SQL] - <insert into Account (accountId, id) values (?, ?)>
2013-03-29 11:52:59,335 DEBUG [org.hibernate.SQL] - <insert into Character (account_id, characterId, wallet_id, id) values (?, ?, ?, ?)>
2013-03-29 11:52:59,335 DEBUG [org.hibernate.SQL] - <insert into Character (account_id, characterId, wallet_id, id) values (?, ?, ?, ?)>
2013-03-29 11:52:59,337 DEBUG [org.hibernate.SQL] - <delete from Character where id=?>

当我们更改account关联的character唯一键characterId时无法删除这个entity,看代码

@Test
@Rollback(false)
public void testSaveAccount(){
    entityManager.persist(account);
    for(Character character: account.getCharacters()){
        if(character.getCharacterId()==1){
            //if change unique key in entity can not remove the entity.
            character.setCharacterId(100L);
            account.getCharacters().remove(character);
        }
    } 
    Assert.assertTrue(account.getCharacters().size()==2);
    entityManager.merge(account);
}

log信息,只是更新了character对象

2013-03-29 12:28:06,364 DEBUG [org.hibernate.SQL] - <insert into Account (accountId, id) values (?, ?)>
2013-03-29 12:28:06,369 DEBUG [org.hibernate.SQL] - <insert into Character (account_id, characterId, wallet_id, id) values (?, ?, ?, ?)>
2013-03-29 12:28:06,369 DEBUG [org.hibernate.SQL] - <insert into Character (account_id, characterId, wallet_id, id) values (?, ?, ?, ?)>
2013-03-29 12:28:06,371 DEBUG [org.hibernate.SQL] - <update Character set account_id=?, characterId=?, wallet_id=? where id=?>

如果我们不去更新这个受管态对象character的状态，则可以删除，使用的时候要注意这个问题。

2.2- One-To-Many 单向和双向关联所产生的不同点

Account和Character单向关联的情况

@Test
@Rollback(false)
public void testSaveBidirection(){
    entityManager.persist(account);
    Character character = account.getCharacters().iterator().next();
    // can not get account by character.
    Assert.assertTrue(character.getAccount()==null);
}

我们可以看到无法从many方取得one方的状态, 下面看双向

@Test
@Rollback(false)
public void testSaveBidirection(){
    // build bidirection relationship.
    for(Character character: account.getCharacters()){
        character.setAccount(account);
    }
    entityManager.persist(account);
    Character character = account.getCharacters().iterator().next();
    // will get account by character.
    Assert.assertTrue(character.getAccount()!=null);
}

开发的时候需要注意，需要建立双向关联才能从子表得到主表的状态。

2.3- One-To-Many 重置子表关系

保存一个Transient状态的Account并重新设置它所关联的character子表,这个account实体已经在数据库存在，看会发生什么

@Test
@Rollback(false)
public void testSaveWhenResetCharacters(){
	// transient state.
    Account account = new Account();
    account.setAccountId(1L);
    
    //Modify the id here: -> fetch the id from databse.
    account.setId(588L);
    
    // transient state.
    Character character3 = new Character();
    character3.setCharacterId(4L);
     
    // reset account-character relationship.
    Set<Character> characters = new HashSet<Character>();
    characters.add(character3);
    account.setCharacters(characters);
    entityManager.merge(account);
}

看log我们只发现了一条sql insert语句，并不会去drop之前关联的characters,因为account不是关系的owner，owner为character

2013-03-29 13:00:21,326 DEBUG [org.hibernate.SQL] - <insert into Character (account_id, characterId, wallet_id, id) values (?, ?, ?, ?)>

再使用@JoinColumn来设置双向关联

    //再account enity中characters属性设置
    @OneToMany(cascade=CascadeType.ALL, orphanRemoval=true)
    @JoinColumn(name="account_id")  //will not generate associated table.

再次执行上面的test case, 看log:

2013-04-01 12:54:54,843 DEBUG [org.hibernate.SQL] - <insert into Character (account_id, characterId, wallet_id, id) values (?, ?, ?, ?)>
2013-04-01 12:54:54,846 DEBUG [org.hibernate.SQL] - <update Character set account_id=null where account_id=? and id=?>
2013-04-01 12:54:54,846 DEBUG [org.hibernate.SQL] - <update Character set account_id=null where account_id=? and id=?>
2013-04-01 12:54:54,847 DEBUG [org.hibernate.SQL] - <update Character set account_id=? where id=?>
2013-04-01 12:54:54,848 DEBUG [org.hibernate.SQL] - <delete from Character where id=?>
2013-04-01 12:54:54,848 DEBUG [org.hibernate.SQL] - <delete from Character where id=?>

因为account和character是双向关联，任意一端都可以维护关系，所以会删除character 子表

Cheers!

461 Words 30 March 2013 Suzhou, China