[SOLVED] How is JPA's Sort.by() is able to access nested fields of entity?

How is JPA's Sort.by() is able to access nested fields of entity?

I am getting a surprising output in my GET API built in springboot, I have two related entities, say A and B. Here A has @OneToOne mapping with B.

@Entity
class A {
    ....
    @OneToOne
    @JoinColumn(name = "b_id")
    B b;
}

@Entity
class B {
    Integer id;
    ....
}

And I am using the JpaRepository for entity A to find the requested entities in specified order.

public interface ARepo extends JpaRepository<A,Integer>, JpaSpecificationExecutor<A> {
   ...
}

and I am building a Pageable like this:

Pageable pageable = PageRequest.of(0, 10, Sort.by(Sort.Direction.DESC, "bId")); // bId i have specified for sorting

So when i execute the findAll() method of JPA i am getting outout in DESC order of b.id. It is so surprising and does my work but I want to know how is it working internally?

I checked for 'nested property path traversal' in hibernate but couldn't find anything about it.

I have a DTO class and a mapper for the same, I doubt that maybe its because this:

@Data
@NoArgsConstructor
@AllArgsConstructor
@Builder
@FieldDefaults(level = AccessLevel.PRIVATE)
@JsonIgnoreProperties(ignoreUnknown = true)
public class ADto {
   Integer aId;
   ....
   Integer bId;
}

@Mapper(componentModel = "spring", nullValueCheckStrategy = NullValueCheckStrategy.ALWAYS)
public interface AMapper {
    @Mapping(target = "bId", source = "b.id")
    ADto entityToDto(A refundInitiate);
}

Solution

The Sort.by(Sort.Direction.DESC, "bId") method uses the expression bId. Spring Data JPA interprets it as a CamelCase path consisting of the b field in the object of class A and the id field of the object that b points to, which is the object of class B.

For more information, see the Spring Data JPA documentation.