Using Hibernate 3.0 on a Postgresql 8.0 db, I need to query for a list of objects that have an empty set in a nested association.

Tables:
tray
rack
sample

Associations:
tray contains 1 or more racks
rack contains 0 or more samples

I need a list of trays that have no samples in any of its racks. I have been partially successful using the following Hibernate criteria query...

emptyTrays = HibernateUtil.getSession().createCriteria(Tray.class)
.createCriteria("racks", "rack")
.add(Expression.isEmpty("rack.samples"))
.list();

The only problem seems to be that there are several copies of the same tray in the list... one for each rack in the tray. For example, if there is 1 empty tray, and that tray has 6 racks, then the resulting list will have 6 Tray objects in it instead of one.

I could do post-processing on the list, but I would like to know if there is a way to use Hibernate to generate a unique list of Tray objects that contain no samples.

Thanks
Eric-

Use the Criteria.setResultTransformer(), using the DistinctRootEntityTransformer class.

Excellent!!!
emptyTrays = HibernateUtil.getSession().createCriteria(Tray.class)
.createCriteria("racks", "rack")
.add(Expression.isEmpty("rack.samples"))
.setResultTransformer(new DistinctRootEntityResultTransformer())
.list();

now returns exactly what I need.

Perhaps a bit easier to remember would be the equivalent form...
.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)


btw,
I didn'tfind any mention of ResultTransformer in 'Hibernate In Action'