jpa - hibernate - how to save parent with detached child

Question

I am sending an object from UI. This object is going to be created with a reference to an existing child.

This is simple illustration for this relation.

class ParentEntity {
    @Id
    Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    private ChildEntity child;
}

class ChildEntity {
    @Id
    Long id;
}



ChildEntity child = new ChildEntity();
child.setId(1);
//parentEntity is created based on data sent from UI
parentEntity.setChild(child);

When I save this object, Hibernate gives me " org.hibernate.TransientPropertyValueException: object references an unsaved transient instance ".

I don't have to save a child as I do not change child at all. Just need to save child's id in parent's table.

I've tried to use few CascadeType, but none of them worked.

Answer 1

Just use the proxy for the child:

parentEntity.setChild(entityManager.getReference(ChildEntity.class, childId));

The point here is to use EntityManager.getReference:

Get an instance, whose state may be lazily fetched.

Hibernate will create the proxy containing only the id without going to the database.