java - 关于 jpa/hibernate subselect 获取的询问

Question

我对代码片段 (full source code) 有疑问这是一本有关 Hibernate/JPA 的书籍随附的代码示例。

这是代码 list :

public class Subselect extends JPATest {

    @Override
    public void configurePersistenceUnit() throws Exception {
        configurePersistenceUnit("FetchingSubselectPU");
    }

    public FetchTestData storeTestData() throws Exception {
        UserTransaction tx = TM.getUserTransaction();
        tx.begin();
        EntityManager em = JPA.createEntityManager();

        Long[] itemIds = new Long[3];
        Long[] userIds = new Long[3];

        User johndoe = new User("johndoe");
        em.persist(johndoe);
        userIds[0] = johndoe.getId();

        User janeroe = new User("janeroe");
        em.persist(janeroe);
        userIds[1] = janeroe.getId();

        User robertdoe = new User("robertdoe");
        em.persist(robertdoe);
        userIds[2] = robertdoe.getId();

        Item item = new Item("Item One", CalendarUtil.TOMORROW.getTime(), johndoe);
        em.persist(item);
        itemIds[0] = item.getId();
        for (int i = 1; i <= 3; i++) {
            Bid bid = new Bid(item, robertdoe, new BigDecimal(9 + i));
            item.getBids().add(bid);
            em.persist(bid);
        }

        item = new Item("Item Two", CalendarUtil.TOMORROW.getTime(), johndoe);
        em.persist(item);
        itemIds[1] = item.getId();
        for (int i = 1; i <= 1; i++) {
            Bid bid = new Bid(item, janeroe, new BigDecimal(2 + i));
            item.getBids().add(bid);
            em.persist(bid);
        }

        item = new Item("Item Three", CalendarUtil.AFTER_TOMORROW.getTime(), janeroe);
        em.persist(item);
        itemIds[2] = item.getId();
        for (int i = 1; i <= 1; i++) {
            Bid bid = new Bid(item, johndoe, new BigDecimal(3 + i));
            item.getBids().add(bid);
            em.persist(bid);
        }

        tx.commit();
        em.close();

        FetchTestData testData = new FetchTestData();
        testData.items = new TestData(itemIds);
        testData.users = new TestData(userIds);
        return testData;
    }

    @Test
    public void fetchCollectionSubselect() throws Exception {
        storeTestData();

        UserTransaction tx = TM.getUserTransaction();
        try {
            tx.begin();
            EntityManager em = JPA.createEntityManager();

            List<Item> items = em.createQuery("select i from Item i").getResultList();
            // select * from ITEM

            for (Item item : items) {
                assertTrue(item.getBids().size() > 0);
                // select * from BID where ITEM_ID in (
                //  select ID from ITEM
                // )
            }

            // The actual test
            em.clear();
            items = em.createQuery("select i from Item i").getResultList();
            // Access should load all collections
            assertTrue(items.iterator().next().getBids().size() > 0);
            em.clear(); // Detach all
            for (Item item : items) {
                assertTrue(item.getBids().size() > 0);
            }

            tx.commit();
            em.close();
        } finally {
            TM.rollback();
        }
    }

}

我不明白的具体部分是这个:

for (Item item : items) {
    assertTrue(item.getBids().size() > 0);
    // select * from BID where ITEM_ID in (
    //  select ID from ITEM
    // )
}

它旨在演示 hibernate 的预取与子选择功能的使用以及将要执行的 SQL 查询。

以下注释://select ID from ITEM 表示所有项目 ID 均从数据库中检索。这就是子选择预取的作用吗？为什么要从 ITEM 表中检索所有 ID？

这是Item实体:

@Entity
public class Item {
 @OneToMany(mappedBy = "item")
 @org.hibernate.annotations.Fetch(
 org.hibernate.annotations.FetchMode.SUBSELECT
 )
 protected Set<Bid> bids = new HashSet<>();
 // ...
}

Answer 1

子选择预取是指在加载关联的子项时，将用于加载父项的原始查询用作子查询，以便一次性初始化所有加载的父项中的子项。您加载父级的查询是select i from Item i(您加载所有项目)，因此它用作出价的子查询(转换为仅投影项目的ID)。

如果您的原始查询是select i from Item i where i.someProperty = :something，则加载出价时的子查询将为select ID from ITEM where SOME_PROPERTY_COLUMN = :something.