Spring Data JPA method for getting the Entity with Joined table

Question

I have very specific question. I have Entity "A" with some variables and OneToMany relations to other table "B". I want to create query to get id of entity "A" and value from table "B" using Locale parameter.

Table B have a translations to entity.

Example:

Entity A:
  id: 1
  var_1: "SDFA"
  OneToMany
  List<B> tabParam;

Entity B:
  id: 2
  value: "ASDF2"
  locale: "en-EN"
  id_A: 1

Entity B:
  id: 3
  value: "ASDF"
  locale: "fr-FR"
  id_A: 1

I want to merge those both table to get for locale "fr-FR":

[ {id: 1, value: "ASDF" }]

Now I got:

[{ id: 1, tabParam: [ {value: "ASDF2"},{value : "ASDF"}]}]

My method from JPA repository:

List<A> findByTabParamLocale(Locale locale)

I am using Locale Hibernate converter from entity to table and from table to entity without any problem.

Answer 1

Base on EntityB.id_A + EntityB.locale is unique. Use bellow projection code to get want you want :

// define the dto interface
public interface LocaleDto {
  Integer getId();
  String getLocale();
}

// define a interface method in your repository
@Query("select a.id, b.locale from EntityA a join a.tabParam b where b.locale = ?1")
List<LocaleDto> findByTabParamLocale(Locale locale)

// or use
@Query("select id_A as id, locale from EntityB where locale = ?1")

Answer 2

Based on your question description

Basically you want to get Id of Entity A. Also your JPA method is look ok only you have need change to getting the Id of Entity A.

Instead of

 List<A> findByTabParamLocale(Locale locale);

Use This

AIdDto findByTabParamLocaleProjectedBy(Locale locale);

AIdDto is projection interface. You will create as follows

public AIdDto {

  Integer getId(); // This is id getter method of Enity A.
}

For more detail. Please refer the spring data jpa reference doc.

https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projections

Through the projection you can try using create custom project value mapper bean like that

Modify AIdDto

 public AIdDto {

  @Value("#{target.id}")
  Integer getId();

  @Value("#{@projectionBean.getLocale(target)}")
  String getLocale();
}

Create ProjectionBean

@Component
class ProjectionBean {

  String getLocale(EntityA entityA) {
    //Write the logic for find the exact EntityB Locale
  }

}

But this is not optimised solution