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

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();