With JPA/JPQL is it possible to manually load and Entity with its OneToMany Association? - spring-boot

Question

Suppose I have 2 entities, Parent and Child, where the parent contains 1..n children:
#Entity
#Data #NoArgsConstructor
public class Parent {
#Id #GeneratedValue
private long id;
private String basic;
private String detail;
#OneToMany(fetch = FetchType.EAGER)
private Set<Child> children = new HashSet<>();
public Parent(String basic, String detail, Set<Child> children) {...}
}
#Entity
#Data #NoArgsConstructor
public class Child {
#Id
#GeneratedValue(strategy = GenerationType.TABLE)
private long id;
private String basic;
private String detail;
public Child(String basic, String detail) {...}
}
I can load Parent-entities by using a JpaRepository:
public interface ParentRepository extends JpaRepository<Parent, Long> { }
// in Controller or Service
List<Parent> parents = parentRepository.findAll();
I'm trying to use projections. For this reason I want to know if it is possible to manually load the parents with a Query, so that i could only load needed data. In a perfect world this could look something like this:
// Dtos, the String detail is not required for both Parent and Child
#Value
public class ParentDto {
long id;
String basic;
Collection<Child> children;
public ParentDto(long id, String basic, Collection<ChildDto> children) {...}
}
#Value
public class ChildDto {
long id;
String basic;
public ChildDto(long id, String basic) {...}
}
and
public interface ParentRepository extends JpaRepository<Parent, Long> {
// Projection - *NOT WORKING*, this is what i would like
#Query("select p.id, p.basic, p.children.id, p.children.basic from Parent p")
List<ParentDto> findAllProjected();
}
This obviously fails, because it will join the all parents with their child, resulting in amount_of_parents * amount_of_their_children rows. The ParentDto would need a contrustor public ParentDto(long id, String basic, ChildDto child) {...}, so i had n ParentDtos per parent where n is the number of children the parent has.
Do i have to manually group the rows by the parents Id's and collect the ChildDtos together? Can I solve this using a subselect? I know Jpa solves this by selecting the parents and doing 1 select for each parent, fetching their children (when i use the autogenerated repository method). I was really hoping that projections/views would be much easier as they are such a fundamental requirement for me and most likely many other applications. Having to always load all data, load only the wanted data - but without their associations or having to load the associations manually for each entity seems like a bumper.
Note: i did try InterfaceProjection but do not want to use it due to it loading all data and stripping away unnessecary parts only when serializing it to json.
Thanks in regards!