Hibernate DTO Projections N+1 查询

Question

Just experimented an interesting fact while using Hibernate & Spring Data JPA. I have the following setup DTO and Entity:

@Getter
@Setter
public class ClassDto {
     private String id;
     private String name;

      public ClassDto(Entity e) {
         this.id = e.id;
        this.name = e.name;
      }

      public ClassDto(String id, String name) {
          this.id = id;
          this.name = name;
      }
}

@Entity
@Setter
@Getter
public class Entity {
     @Id String id;
     String name;
}

Why this query got executed N+1 times.

select new org.xyz.ClassDto(e) from Entity e

While this one with entity destructured just once

select new org.xyz.ClassDto(e.id, e.name) from Entity e

I want only a logical explanation about how things works underground.

Thanks

Answer 1

I guess that you are using a query like this:

select new org.xyz.ClassDto(e.toManyAssociation) from Entity e

In this case, Hibernate will pass in a proxy instead of a fully initialized entity, so when you access the data of that object in the constructor, Hibernate will lazy load the object. You can solve that by joining the association and passing the join alias instead.

I think you might be interested inBlaze-Persistence Entity Views though.

I created the library to allow easy mapping between JPA models and custom interface or abstract class defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure(domain model) the way you like and map attributes(getters) via JPQL expressions to the entity model.

A DTO model for your use case could look like the following with Blaze-Persistence Entity-Views:

@EntityView(Entity.class)
public interface ClassDto {
    @IdMapping
    Long getId();
    String getName();
}

Querying is a matter of applying the entity view to a query, the simplest being just a query by id.

ClassDto a = entityViewManager.find(entityManager, ClassDto.class, id);

The Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features

Page<ClassDto> findAll(Pageable pageable);

The best part is, it will only fetch the state that is actually necessary!