How do I define a custom JOIN condition in a query method of a JpaRepository - spring

Question

We have an ArticleEntity that has several Prices, and the Prices are only valid in a certain time range, and there is only one valid price at a time.
In our entities this is modeled:
#Entity
public class ArticleEntity {
...
#OneToMany(mappedBy = "article", fetch = FetchType.LAZY)
private Collection<PriceEntity> prices;
}
Now, in our ArticleRepository I am trying to add custom query so that only valid prices are retrieved:
public interface ArtikelRepository extends JpaRepository<ArtikelEntity, Integer> {
#Query(value = "select a, p from ArtikelEntity a left join fetch PriceEntity p on p.article = a " +
"and (p.start is null or p.start <= :date) " +
"and (p.end is null or p.end >= :date) ")
Collection<ArtikelEntity> findAllWithPriceOnDate(#Param("date") LocalDate date);
}
Unfortunately this does not work, as Hibernate/JPA is requesting all prices afterwards again, instead of populating the price list with just the result of this join query.
Can I tell Hibernate/JPA to use the data from the custom query? And how?

Answer 1

It's not really possible unless you use Hibernate filters. I would recommend you to look into a DTO solution though, because even with filters, you could run into a situation where you change the collection and all of a sudden Hibernate flushes the state i.e. removes elements which are filtered out.
I think this is a perfect use case for Blaze-Persistence Entity Views.
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(ArtikelEntity.class)
public interface ArtikelDto {
#IdMapping
Long getId();
String getName();
#Mapping("prices[(start is null or start <= :date) " +
"and (end is null or end >= :date)]")
Set<PriceDto> getValidPrices();
#EntityView(PriceEntity.class)
interface PriceDto {
#IdMapping
Long getId();
BigDecimal getPrice();
}
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
ArtikelDto a = entityViewManager.find(entityManager, ArtikelDto.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<ArtikelDto> findAll(#OptionalParam("date") LocalDate date, Pageable pageable);
The best part is, it will only fetch the state that is actually necessary!