如何抓取平行集合

join抓取两个以上的平行集合会产生笛卡尔积，而如果使用subselect抓取又会失去动态抓取集合其他依赖对象的机会。要怎样才能动态地抓取平行集合呢？目前我能想到的一种方法就是生成多条select，分别抓取平行集合。我觉得这并不是一个很完美的方案，但是它却实是可行的。

以Forum为例，在某个use case中我们需要加载一个Forum，同时要显示它的所有Thread以及Moderator.Forum的类代码为：

@Entity @Table public class Forum implements ForumNode,Serializable{ @ManyToMany private Set<User> moderators; @OneToMany private List<Thread> threads = new ArrayList<Thread>(); }  

根据ID加载一个Forum，并将这两个集合抓取出来的方法是：

public Forum getForumWithModeratorsAndThreads(final long forumId){ return (Forum) getHibernateTemplate().executeWithNativeSession(new HibernateCallback() { public Object doInHibernate(Session session) throws HibernateException, SQLException { //Get forum. String getForuumHql= "from Forum as forum where forum.id=:forumId"; Forum forum = (Forum) session.createQuery(getForuumHql).setParameter("forumId", forumId).uniqueResult(); //Fetch threads. String fetchForumThreadsHql = "from Forum as forum left join fetch forum.threads as thread left join fetch thread.subject where forum=:forum"; session.createQuery(fetchForumThreadsHql).setParameter("forum", forum).list(); //Fetch moderators. String fetchForumMorderatorsHql = "from Forum as forum left join fetch forum.moderators as moderator left join fetch moderator.roles where forum=:forum"; session.createQuery(fetchForumMorderatorsHql).setParameter("forum", forum).list(); logger.info("The Forum (ID="+forumId+") has loaded with moderators and threads"); return forum; } }); }  

这段代码做了三个操作，生成三条SQL.第一个操作是加载一个不带任何关联对象的Forum。第二个操作是抓取这个Forum的Thread集合。第三个操作是抓取这个Forum的Moderator集合。

下面是测试代码：

@Test @Transactional public void testgetForumWithModeratorsAndThreads(){ Forum forum = forumRepository.getForumWithModeratorsAndThreads(1L); System.out.println("****************************************************"); System.out.println(forum.getModerators().size()); System.out.println(forum.getThreads().size()); }

 

测试的输出结果是：

Hibernate:
    /*
from
    Forum as forum
where
    forum.id=:forumId */ select
        forum0_.id as id2_,
        forum0_.creationTime as creation2_2_,
        forum0_.description as descript3_2_,
        forum0_.groupId as groupId2_,
        forum0_.modifiedTime as modified4_2_,
        forum0_.name as name2_
    from
        Forum forum0_
    where
        forum0_.id=?
Hibernate:
    /*
from
    Forum as forum
left join
    fetch forum.threads as thread
left join
    fetch thread.subject
where
    forum=:forum */ select
        forum0_.id as id2_0_,
        threads1_.id as id5_1_,
        post2_.id as id4_2_,
        forum0_.creationTime as creation2_2_0_,
        forum0_.description as descript3_2_0_,
        forum0_.groupId as groupId2_0_,
        forum0_.modifiedTime as modified4_2_0_,
        forum0_.name as name2_0_,
        threads1_.creationTime as creation2_5_1_,
        threads1_.forumId as forumId5_1_,
        threads1_.modifiedTime as modified3_5_1_,
        threads1_1_.subjectId as subjectId6_1_,
        threads1_.forumId as forumId0__,
        threads1_.id as id0__,
        post2_.authorId as authorId4_2_,
        post2_.creationTime as creation2_4_2_,
        post2_.isSubject as isSubject4_2_,
        post2_.messageBody as messageB4_4_2_,
        post2_.modifiedTime as modified5_4_2_,
        post2_.quotedPostId as quotedPo8_4_2_,
        post2_.threadId as threadId4_2_,
        post2_.title as title4_2_
    from
        Forum forum0_
    left outer join
        Thread threads1_
            on forum0_.id=threads1_.forumId
    left outer join
        Thread_Subject threads1_1_
            on threads1_.id=threads1_1_.threadId
    left outer join
        Post post2_
            on threads1_1_.subjectId=post2_.id
    where
        forum0_.id=?
Hibernate:
    /*
from
    Forum as forum
left join
    fetch forum.moderators as moderator
left join
    fetch moderator.roles
where
    forum=:forum */ select
        forum0_.id as id2_0_,
        user2_.id as id0_1_,
        role4_.id as id1_2_,
        forum0_.creationTime as creation2_2_0_,
        forum0_.description as descript3_2_0_,
        forum0_.groupId as groupId2_0_,
        forum0_.modifiedTime as modified4_2_0_,
        forum0_.name as name2_0_,
        user2_.accountNonExpired as accountN2_0_1_,
        user2_.accountNonLocked as accountN3_0_1_,
        user2_.credentialsNonExpired as credenti4_0_1_,
        user2_.email as email0_1_,
        user2_.enabled as enabled0_1_,
        user2_.password as password0_1_,
        user2_.username as username0_1_,
        user2_.version as version0_1_,
        moderators1_.forumId as forumId0__,
        moderators1_.moderatorId as moderato2_0__,
        role4_.description as descript2_1_2_,
        role4_.name as name1_2_,
        roles3_.userId as userId1__,
        roles3_.roleId as roleId1__
    from
        Forum forum0_
    left outer join
        Forum_Moderator moderators1_
            on forum0_.id=moderators1_.forumId
    left outer join
        User user2_
            on moderators1_.moderatorId=user2_.id
    left outer join
        User_Role roles3_
            on user2_.id=roles3_.userId
    left outer join
        Role role4_
            on roles3_.roleId=role4_.id
    where
        forum0_.id=?
INFO - ForumHibernateRepository$1.doInHibernate(38) | The Forum (ID=1) has loaded with moderators and threads
****************************************************
1
2

 

从运行结果来看，getForumWithModeratorsAndThreads方法返回的Forum是一个已经将moderator和thread抓取出来的Forum实例。当我们在测试代码中访问这两个集合时，并没有任何sql打出，说明它们都已经被抓取出来了。

 

我觉得这个方案不好的地方在于抓取两个平行集合的hql有些怪异，它们select的是Forum，但实际上我们并不是要这个Forum,而是要抓取它的集合。因此执行些hql后，我们也没有要得到查询结果的意思。另外，每次查询都要从Forum开始进行join也觉得多做一些事情。 通过验证我发现， 如果直接select这个forum的thread和moderator集合，再访问这个forum的这两个集合时，它还是要去查DB，这说明， 直接select它们不会导致forum的 thread和 moderator集合（应该是hibernate的colletcion wapper）被加载。我想原因是因为hibernate无法确认我们通过select查出的这些元素一定是forum的全部thread集合。hibernate必须重新执行一次以确保全部的集合被加载出来。因此目前来说，也只有这种方式还算可行。我想以后可能会找到更好的解决方法吧。