java如何使用带@OneToOne关系的fetch join？

Question

我有实体贴子，贴子细节。 这些是一对一的关系，PostDetail是可选的

class Post {
    @Id
    private int id;

    @Column
    private String title;

    @Column
    private Strint contents;

    @OneToOne(cascade = CascadeType.ALL)
    @PrimaryKeyJoinColumn
    private PostDetail postDetail;
}

class PostDetail {
    @Id
    private int id;

    @Column
    private boolean duplicated;

    @OneToOne
    private Post post;
}

public interface PostRepository extends JpaRepository<Post, Integer> {
    @Transactional
    @Query("SELECT a FROM Post a LEFT JOIN FETCH a.postDetail")
    public Page<Post> getAll(Example<Post> example, Pageable pageable);

    @Transactional
    @Query("SELECT a FROM Post a LEFT JOIN FETCH a.postDetail")
    public List<Post> getAll();
}

当应用程序启动时，会发生异常

Caused by: org.hibernate.QueryException: query specified join fetching, but the owner of the fetched association was not present in the select list ...

有什么问题吗？我尝试这样做是为了避免在查询帖子列表（getAll（））时出现N+1问题

---

抱歉，我修改了我的问题

这两种PostRepository方法都会出错

第一个getAll（）抛出错误“查询指定的联接获取…”

第二个getAll（）抛出错误

org.mariadb.jdbc.internal.common.QueryException: Unknown column 'postdetail1_.post_id' in 'field list'

Answer 1

如果想避免1+N查询问题，可以使用EntityGraph。只需在自定义Repo中重写'findAll'方法，并对其使用EntityGraph注释，如下所示：

public interface PostRepository extends JpaRepository<Post, Integer> {

    @Override
    @EntityGraph(attributePaths = {"postDetail"})
    Page<Post> findAll(Pageable pageable);

    @Override
    @EntityGraph(attributePaths = {"postDetail"})
    List<Post> findAll();
}

Answer 2

因为您正在使用Spring Data JPA，所以Post中id的类型应该与JpaRepository<T, ID extends Serializable>中的ID一致，只需做一些修改：

@Entity
@Table(name = "post")
class Post {
    @Id
    private Integer id;

    @Column
    private String title;

    @Column
    private String contents;

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "postDetail_id")//assume this is the name
    private PostDetail postDetail;
}

您不必为getAll()定义查询，Spring Data JPA为常见查询提供了一些嵌入式实现，只需调用它们：

public interface CrudRepository<T, ID extends Serializable> extends Repository<T, ID> {

    <S extends T> S save(S entity); 

    T findOne(ID primaryKey);       

    Iterable<T> findAll();          

    Long count();                   

    void delete(T entity);          

    boolean exists(ID primaryKey);  

    // … more functionality omitted.
}

所以PostRepository就像：

public interface PostRepository extends JpaRepository<Post, Integer> {

}

然后可以在单元测试中测试它：

@RunWith(SpringRunner.class)
@SpringBootTest
public class MyTest {

    @Autowired
    private PostRepository repo;

    @Test
    public void testFindAll(){
        repo.findAll();
    }

    @Test
    public void testFindById(){
        repo.findOne(id);
    }
}