JPA cascade persist and references to detached entities throws PersistentObjectException. Why?

I have an entity Foo that references an entity Bar:

@Entity
public class Foo {

    @OneToOne(cascade = {PERSIST, MERGE, REFRESH}, fetch = EAGER)
    public Bar getBar() {
        return bar;
    }
}

When I persist a new Foo, it can get a reference to either a new Bar or an existing Bar. When it gets an existing Bar, which happens to be detached, my JPA provider (Hibernate) throws the following exception:

Caused by: org.hibernate.PersistentObjectException: detached entity passed to persist: com.example.Bar
 at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:102)
 at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:636)
 at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:628)
 at org.hibernate.engine.EJB3CascadingAction$1.cascade(EJB3CascadingAction.java:28)
 at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:291)
 at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:239)
 at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:192)
 at org.hibernate.engine.Cascade.cascade(Cascade.java:153)
 at org.hibernate.event.def.AbstractSaveEventListener.cascadeBeforeSave(AbstractSaveEventListener.java:454)
 at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:288)
 at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:204)
 at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:130)
 at org.hibernate.ejb.event.EJB3PersistEventListener.saveWithGenerat开发者_开发技巧edId(EJB3PersistEventListener.java:49)
 at org.hibernate.event.def.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:154)
 at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:110)
 at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:61)
 at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:645)
 at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:619)
 at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:623)
 at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:220)
 ... 112 more

When I either make sure the reference to Bar is managed (attached) or when I omit the cascade PERSIST in the relation, all works well.

Neither solution however is 100% satisfactory. If I remove the cascade persist, I obviously can't persist a Foo with a reference to a new Bar anymore. Making the reference to Bar managed necessitates code like this prior to persisting:

if (foo.getBar().getID() != null && !entityManager.contains(foo.getBar())) {
    foo.setBar(entityManager.merge(foo.getUBar()));
}
entityManager.persist(foo);

For a single Bar this might not seem like a big deal, but if I have to take all properties into account like this I'll end up with pretty horrible code that seems to defeat the reason of using ORM in the first place. I might as well well persist my object graph manually using JDBC again.

When given an existing Bar reference the only thing JPA has to do is take its ID and insert that in a column of the table that holds Foo. It does exactly this when Bar is attached, but throws the exception when Bar is detached.

My question is; why does it need Bar to be attached? Surely its ID won't change when the Bar instance transitions from detached to attached state, and that ID seems to be the only thing needed here.

Is this perhaps a bug in Hibernate or am I missing something?

You can use merge() instead of persist() in this case:

foo = entityManager.merge(foo); 

When applied to the new instance, merge() makes it persistent (actually - returns the persistent instance with the same state), and merges cascaded references, as you try to do manually.

If I understand correctly, you just need the Bar reference to allow the new Foo to have the foreign key value (to the existing Bar) when persisting. There is a JPA method on the EntityManager called getReference() that might be useful to you for this case. The getReference() method is similar to find() except that it won't bother to return a managed instance (of Bar) unless it happens to already be cached in the persistence context. It will return a proxy object that will satisfy your foreign key needs in order to persist the Foo object. I'm not sure if this is the kind of solution you were hoping for, but give it a try and see if this works for you.

I also noticed from your code that you're using "property" style access instead of "field" style access by annotating your getter method (for the Bar relationship). Any reason for that? It's recommended that you annotate the members rather than the getters for performance reasons. It is supposed to be more efficient for the JPA provider to access the field directly rather than via getters and setters.

EDIT:

As someone else mentioned, using a cascade merge() will persist new entities as well as merge modified entities and reattach detached entites that have a relationship with the MERGE cascade option. Using PERSIST cascade option won't reattach anything or merge anything and is meant to be used when that is the behavior you want.