How to retrieve a complex class and its members using Hibernate Projection?

Question

I have a class as following that need to retrieve from DB using Hibernate. The problem is my class has multiple members and majority of them are classes, how can I retrieve them?

@Entity
public class Student {
  @Id
  long id;
  String name;
  String fname;
  @OneToMany
  List<Course> courses;
  @ManyToOne
  Dealer dealer;
  ...
}

@Entity
public class Dealer {
   @Id
   long id;
   String name; 
   @OneToMany(fetch = FetchType.LAZY, mappedBy = "cr.dealer", cascade = CascadeType.ALL)
   Set<Car> cars = new HashSet<Cars>(0);
   ..

}

I need to retrieve student id 1 and all its courses, its dealer and list of dealers' cars.

My projection is as following but it does not return anything.

  ...
    .setProjection(Projections.projectionList()

    .add(Projections.property("friends.cars").as("cars")
    ...

Answer 1

Because you have a List of Courses and a Set of Cars, you can simply fetch the whole graph in a single query:

select s
from Student s
left join fetch s.courses
left join fetch s.dealer d
left join fetch d.cars
where s.id = :id

Because you are fetching two collections, this query will generate a Cartesian Product, so you need to make sure that the selected children collections don't have too many entries.

If you don;t want to run into a Cartesian product, you can simply run this query:

select s
from Student s
left join fetch s.courses
left join fetch s.dealer d
where s.id = :id

and then you access the dealer.cars to fetch that collection with a separate query:

Student s = ...;
s.getDealer().getCars().size();