Spring-Data 中的父对象不获取子对象

Question

我对 Hibernate 很陌生，我已经在父对象和子对象之间设置了 @OneToMany 双向关系，并期望当我将所有内容保存在数据库中时，将父对象设置为子对象（因为子对象是关系），当我使用 ORM 获取父对象时，我希望也获取一个子对象，但作为回报，我看到零个子对象。

据我在互联网上阅读的内容所了解的，当我将子对象作为与父级的关系的所有者时，完整的 ORM 应该会根据预定义的关系映射来连接两个对象.这些信息正确吗？

@Entity
@Table(name = "stream")
@SequenceGenerator(name = "sequence", sequenceName = "stream_id", allocationSize = 1)
public class StreamEntity extends EntityWithId {

    // @MappedSuperclass with ID 

    @Column(name = "request_id")
    @OneToMany(
            mappedBy = "streamEntity",
            fetch = FetchType.LAZY
    )
    private List<RequestEntity> requestEntities = new ArrayList<>();

    // getters setters
}

@Entity
@Table(name = "request")
@SequenceGenerator(name = "sequence", sequenceName = "request_id", allocationSize = 1, initialValue = 1)
public class RequestEntity extends EntityWithId {

    // @MappedSuperclass with ID 

    @JoinColumn(name = "stream_id")
    @ManyToOne(fetch = FetchType.LAZY)
    private StreamEntity streamEntity;

    // getters setters
}

一些测试：

@Test
public void createStreamWithSeveralStreamTargetsAndDeleteStream() {
    List<StreamEntity> streamEntities = createStream(1); // stream object is persisted here

    List<RequestEntity> requestEntitySave = createRequest(1, streamEntityCheck);
    assertEquals(1, requestEntitySave.size()); // request entity is persisted with the stream object into the database

    List<StreamEntity> streamEntities = streamDao.findAll();
    assertEquals(1, streamEntities.size());
    StreamEntity streamEntity2 = streamEntities.get(0);
    streamEntity2.getRequestEntities(); - this collection shows 0
}

我错过了什么？

更新： 正如@Alan Hay 建议的那样，我已经在我的测试中添加了一个@Persistent 上下文，现在它看起来像这样：

@PersistenceContext
    private EntityManager em;


@Test
    public void createStreamWithSeveralStreamTargetsAndDeleteStream() {
        List<StreamEntity> streamEntities = createStream(1); // stream object is persisted here

        List<RequestEntity> requestEntitySave = createRequest(1, streamEntityCheck);
        assertEquals(1, requestEntitySave.size()); // request entity is persisted with the stream object into the database

        em.clear();

List<RequestEntity> requestEntities = requestDao.findAll();
        assertEquals(1, requestEntities.size()); // retirns zero objects

        List<StreamEntity> streamEntities = streamDao.findAll();
        assertEquals(1, streamEntities.size());
        StreamEntity streamEntity2 = streamEntities.get(0);
        streamEntity2.getRequestEntities();// this collection shows 0
    }

完成此操作后，我在 requestDao.findAll(); 上得到零个对象；步。我已在上一步中提交了这些更改，为什么我无法获取此实体？

Answer 1

由于一级缓存的概念，您创建和保存的初始对象与您加载的对象是同一个实例。由于您没有将创建的请求实体添加到内存模型中，而只是将其持久化到数据库中，因此内存模型中不存在关系。

要获得所需的结果，您需要清除保存和重新加载之间的持久性上下文。

@Test
    public void createStreamWithSeveralStreamTargetsAndDeleteStream() {
        List<StreamEntity> streamEntities = createStream(1); 

        List<RequestEntity> requestEntitySave = createRequest(1, streamEntityCheck);
        assertEquals(1, requestEntitySave.size()); 

        //you need to clear the persistence context here
        //now a db query should execute.
        //Inject the entitymanager to your test @PersistenceContext EntityManager em;
        em.clear();

        List<StreamEntity> streamEntities = streamDao.findAll();
        assertEquals(1, streamEntities.size());
        StreamEntity streamEntity2 = streamEntities.get(0);
        streamEntity2.getRequestEntities(); - this collection shows 0

   }