带连接的 JPA 标准构建器包装器

Question

How can I build a DTO using JPA Criteria builder with a join on tables linked by a one-to-many relation?

In the documentation, there is no example using both a wrapper and join cases.

JPA Doc

For instance :

EntityA {
   String name;

   @OneToMany
   Set<EntityB> items;

   ...
}


Wrapper {
   name;
   Set<EntityB> items;
}

Answer 1

If I remember right, you can't. Projection doesn't handle joins. Perhaps you might want to query a list of EntityB instead of an EntityA with items and pass the items list to a Dto object that takes the parent entity and its list. Not what you want, to be sure, but should get the job done. So, by example:

@Entity
public class EntityA {
    @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

    @OneToMany(mappedBy="a")
    private Set<EntityB> bs;

@Entity
public class EntityB {
    @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    private EntityA a;

public class WrapperDto {
    private EntityA a;
    private List<EntityB> bs;
    public WrapperDto(EntityA a, List<EntityB> bs) {
        this.a = a;
        this.bs = bs;
    }

and to use it:

    tx.begin();
    EntityA a = new EntityA();
    EntityB b1 = new EntityB(); 
    EntityB b2 = new EntityB();
    b1.setA(a);
    b2.setA(a);
    em.persist(a);
    em.persist(b1);
    em.persist(b2);
    tx.commit();
    em.clear();

//  projection with join fetch doesn't work.  
//  em.createQuery("select new dto.WrapperDto( a, bs ) from EntityA a left outer join fetch a.bs bs where a.id = 1", WrapperDto.class).getResultList();

//  a possible solution
    EntityA aFound = em.find(EntityA.class, 1L);
    List<EntityB> bs = em.createQuery("select b from EntityB b where b.a = :a", EntityB.class).setParameter("a", aFound).getResultList();
    WrapperDto dto = new WrapperDto(aFound, bs);