Question

I have two entities, Ablum and Image, which are in many to many relationship.
I wanna make a criteria query that to get all Albums and the counts on how many Images they have.
I don’t want to get all Albums first then loop the result to get the counts as there would be so many sql requests.
I’ve been working for 2 nights and complete lost. If cannot find a way out maybe I need to fallback to use SQL.

Answer 1

Thanks to digitaljoel’s inspiration, I found that CriteriaBuilder has a method call “size” that can be put on collections. Below is the code:

    CriteriaBuilder cb = getCriteriaBuilder();
    CriteriaQuery<Object[]> query = cb.createQuery(Object[].class);
    Root<AlbumEntity> albums = query.from(AlbumEntity.class);
    query.select(cb.array(albums.get(AlbumEntity_.id), cb.size(albums.get(AlbumEntity_.images))));
    query.groupBy(albums.get(AlbumEntity_.id));

Here the groupBy call is a must otherwise error will occur.
But this method is to load the IDs of AlbumEntity, not the entity itself. The Album entity can be load if below code is used:

    query.select(cb.array(albums, cb.size(albums.get(AlbumEntity_.images))));
    query.groupBy(albums.get(AlbumEntity_.id), ...);

The groupBy must include all properites of the album entity. And it still does not work if the album entity has blob type property.