Question

I have two entities: parent Customer and child Order.
Each Customer has 1,000,000 Orders for example, so it is not needed in any given time to load a Customer with all Orders but I want to have this ability to make join query on these two entities in JPA.
So because of this, I must create @OneToMany relationship for making join queries.

My question is: how to get query without making joinColumn because even in Lazy mode it is possible to load 1,000,000 objects!
I just want to get query on these object with where restrictions like native join.

Answer 1

If you don’t want the @OneToMany relationship implicitly set in your Customer class than you don’t have to. You can execute JPQL queries (in very precise manner) without the marked relationship.

Assume you have:

@Entity
public class Customer {

    // all Customer-related fields WITHOUT @OneToMany relationship with Order
}

@Entity
public class Order {

    @ManyToOne
    private Customer owner;
}

Then if you want to get all Orders for particular Customer you can execute a simple JPQL query like that:

// Customer customer = ...
// EntityManager em = ...

String jpql = "SELECT o FROM Order o WHERE o.owner = :customer";

TypedQuery<Order> query = em.createQuery(jpql, Order.class);
query.setParameter("customer", customer);

List<Order> orders = query.getResultList();

In this way you can execute the code only when you’re really sure you want to fetch Customer's orders.

I hope I’ve understood your problem correctly.