Efficiently determining the IDs of entities referenced via OneToMany relationship

Question

Let's say I have a Hibernate entity that declares a OneToMany relationship to a different entity:

@Entity
public class SomeEntity {
  @OneToMany(fetch = FetchType.LAZY)
  private List<OtherEntity> otherEntities = new LinkedList<OtherEntity>();

  [...]
}

When mapping SomeEntity to the corresponding DTO, all I need are the IDs that identify OtherEntity as primary key (i.e., I am not actually interested in OtherEntity instances).

Does Hibernate support this pattern, i.e., only retrieving the IDs of entities referenced via a OneToMany relationship?

I cannot influence how SomeEntity is retrieved (i.e., I have an existing SomeEntity instance retrieved within te scope of the current Hibernate session), but let's assume that lazy loading has not yet taken place, so just retrieving the child objects' IDs (rather than the complete objects) would actually yield a performance benefit.

Answer 1

Well, if you only need the entities' ids and you want to be economical about it, when you get those entities from the database you should state in your query that you only want to get the ids of each entry, using projections, something like :

 SELECT Entity.id as entity FROM Entity WHERE ... 

This will return an array of objects of the same type as Entity's id field type.