JPA: @OrderBy bypassed by cache?

[Per Lindberg]

It appears that the @OrderBy annotation on a @OneToMany LinkedList member of an @Entity does not work when another item is added last to the list by a @Stateless session bean. The next access (by later HTTP request to a @Stateless session bean) calling myEntityManager.find will see an unsorted list. Restarting the app server solves the problem. My guess is that the JPA/ORM engine keeps a cache and don't give a hoot about @OrderBy. Example:

@Entity
public class CarMaker {
int id;
private List<CarModel> carModels = new LinkedList<CarModel>();
...
@OneToMany(mappedBy="carMaker")
@OrderBy("name")
public List<CarModel> getCarModels() {
return carModels;
}
...
}
@Entity
public class CarModel implements HasId {
int id;
private String name;
...
@ManyToOne
@JoinColumn(name="car_maker_id", referencedColumnName="id")
public CarMaker getCarMaker() {
return carMaker;
}
...
}
@Stateless
public class CarModelEjb
extends GenericEjb<CarModel, Integer>
implements CarModelLocal
{
@EJB private CarMakerLocal carMakerLocal;
public CarModel create(int carMakerId, String name) {
CarMaker maker = carMakerLocal.find(carMakerId);
CarModel model = new CarModel();
model.setName(name);
model.setCarMaker(maker);
persist(model);
maker.getCarModels().add(model);
return model;
}
...
}

The first request does a carModelLocal.create(carMakerId, name, year);

The second request does a CarMaker carMaker = carMakerLocal.find(carMakerId)); which returns em.find(persistentClass, id);

The resulting carMaker has the new CarModel last in the LinkedList, ignoring @OrderBy. Aarrgh!

As I said, the probable culprit is some cache in the JPA/ORM engine. I use Glassfish v2.

Is this a bug or a feature?

What would be a canoncial workaround? Adding "manual" sorting in getCarModels()? Or is there a better way?

[James Sutherland]

If you want to have the List ordered you need to maintain the order when you add to it. In your addCarModel() method in your object you can use Collections.sort() to fix the order after adding to it.

You could also use a TreeSet to automatically maintain the order, but this would not work with a LAZY mapping.

In JPA 2.0 there is also support for an OrderColumn which maintains the order in the database as define in the object model.

See also,
http://en.wikibooks.org/wiki/Java_Persistence/Relationships#Ordering

To disable caching in TopLink Essentials (default provider in Glassfish 2), you can use "toplink.cache.shared.default"="false" in your persistence.xml.

You can also call refresh() on the object, to refresh it from the database.

[Per Lindberg]

Thanks James! Crystal clear explanation.

It appears that the best solution in this case is to add Collartions.sort() to all business methods that inserts (and renames!) List members. Re-sorting on each access seems ineffective. Ditto disabling the cache or calling refresh().