一对一映射正在触发 N+1 个查询

Question

I'm trying to make an @OneToOne mapping following the https://vladmihalcea.com/the-best-way-to-map-a-onetoone-relationship-with-jpa-and-hibernate/ the mapping itself works but its triggering an N+1 query problem.

The query is being made on the parent entity service and its triggering N+1 queries. How can I improve this code to only make 1 query? We don't need to access the ParentDetails in this case.

EDIT: I've tried using JPQL and LEFT JOIN FETCH ParentDetails and didn't work either.

EDIT2: Just to try to add more information. I've put a breakpoint on the getParentDetails just to make sure I was not calling the getter anywhere and I'm not calling and double-checked and it seems a join problem on the repo call.

Let's go to the code:

Parent

@Entity
@DynamicUpdate
public class Parent {

   @Id
   @GeneratedValue(strategy = GenerationType.AUTO)
   private Long id;
   @OneToOne(fetch = FetchType.LAZY,
        cascade = CascadeType.ALL,
        mappedBy = "parent")
   private ParentDetails parentDetails;

 // Getters, setters, etc omitted for brevity
}

ParentDetails

@Entity
public class ParentDetails {

    @Id
    private Long id;
    @OneToOne(fetch = FetchType.LAZY)
    @MapsId
    private Parent parent;

    // Getters, setters, etc omitted for brevity

ParentDetailsRepository

@Repository
public interface ParentRepository extends JpaRepository<Parent, Long> {

    Page<Parent>findByNameOrderByName(@Param("name") final String name,final Pageable pageable);
}

Answer 1

Hibernate executes the additional queries because the Parent entity doesn't map the foreign key column. Hibernate doesn't support lazy fetching for that end of the association. When Hibernate instantiates a Parent object, it needs to check if it needs to initialize the association with a proxy or a null object. And at some point, the team decided that they would fetch the associated entity if they are forced to perform a query anyways. I explained that in more detail here: https://thorben-janssen.com/hibernate-tip-lazy-loading-one-to-one

If you want to avoid the additional queries, you need to model a unidirectional association between ParentDetails and Parent. In your example, that would mean that you need to remove the parentDetails attribute from your Parent entity.

@Entity
@DynamicUpdate
public class Parent {

   @Id
   @GeneratedValue(strategy = GenerationType.AUTO)
   private Long id;

 // Getters, setters, etc omitted for brevity
}

Because your ParentDetails entity uses the same id value as the Parent entity, you don't need a bidirectional association. If you want to get the ParentDetails entity for a Parent entity, you can get it with a call of the em.find(...) method

Parent p = // fetch the parent object ... 
ParentDetails pd = em.find(p.getId(), ParentDetails.class);