Hibernate one-to-one: getId() without fetching entire object

I want to fetch the id of a one-to-one relationship without loading the entire object. I thought I could do this using lazy loading as follows:

class Foo { 
    @OneToOne(fetch = FetchType.LAZY, optional = false)
    private Bar bar; 
}


Foo f = session.get(Foo.class, fooId);  // Hibernate fetches Foo 

f.getBar();  // Hibernate fetches full Bar object

f.getBar().getId();  // No further fetch, returns id

I want f.getBar() to not trigger another fetch. I want hibernate to give me a proxy object that allows me to call .getId() without actually fetching the Bar object.

What am I doing wrong?

Solution 1:

Use property access strategy

Instead of

@OneToOne(fetch=FetchType.LAZY, optional=false)
private Bar bar;

Use

private Bar bar;

@OneToOne(fetch=FetchType.LAZY, optional=false)
public Bar getBar() {
    return this.bar;
}

Now it works fine!

A proxy is initialized if you call any method that is not the identifier getter method. But it just works when using property access strategy. Keep it in mind.

See: Hibernate 5.2 user guide

Solution 2:

Just to add to the Arthur Ronald F D Garcia'post: you may force property access by @Access(AccessType.PROPERTY) (or deprecated @AccessType("property")), see http://256stuff.com/gray/docs/misc/hibernate_lazy_field_access_annotations.shtml

Another solution may be:

public static Integer getIdDirect(Entity entity) {
    if (entity instanceof HibernateProxy) {
        LazyInitializer lazyInitializer = ((HibernateProxy) entity).getHibernateLazyInitializer();
        if (lazyInitializer.isUninitialized()) {
            return (Integer) lazyInitializer.getIdentifier();
        }
    }
    return entity.getId();
}

Works for detached entities, too.