How can I do paging with @OneToMany collections

Question

Suppose I have a Post entity and a Comment entity and a one to many relationship:

@Entity class Post {
    ...
    @OneToMany
    List<Comment> comments;
    ...
}

How can I achieve paging like this:

Post post = //Find the post.
return post.getComments().fetch(100, 10); // Find the 11th page (page size 10);

Is it possible to emulate dynamic paging with @OneToMany collections on top of JPA, or do we have to rewrite the association mechanism of JPA totally ? (e.g. create a PersistentList collection type that could manage the paging, sorting and searching).

P.S.: I recently found the Play! framework uses a very interesting lib on top of JPA: Siena. Siena is very easy to use, and is a good abstraction on top of JPA/Hibernate. But I can't find how to do paging with its associations.

Update:

Play framework has a query syntax similar to Django:

Post.findAll().from(100).fetch(10);  // paging

where

Post.findAll() 

will return a JPAQuery object, a customized query type in Play.

But with associated collections, e.g.:

Post.comments

will just return a List, which doesn't support paging or other queries.

I was wondering how to extend it, so that

Post.comments

will also return a JPAQuery object or similar, then you can query on the "query" collection:

Post.comments.from(100).fetch(10);

or insert a new Comment without actually fetching any of the comments:

Post.comments.add(new Comment(...));

On my first thought, we could create a subclass of List, then the Post class would become:

@Entity class Post {
    ...
    @OneToMany
    QueryList<Comment> comments;
    ...
}

and QueryList will have fetch(), from() methods that indirect to JPAQuery's.

But I don't know whether Hibernate/JPA will recognize this, or interfere with it.

Answer 1

Is it possible to emulate dynamic paging with @OneToMany collections on top of JPA (...)

Not supported. The standard approach would be to use a JPQL query to retrieve the comments for a given post and and to use Query#setFirstResult(int) and Query#setMaxResults(int).

On my first thought, we could create a subclass of List, (...). But I don't know whether Hibernate/JPA will recognize this, or interfere with it.

It obviously won't without an heavy patch to drastically change the default behavior.

Answer 2

I think the "right" way might be more like:

@Entity
class Post {
    ...

    public GenericModel.JPAQuery getComments() {
        return Comment.find("post_id = ?", post_id);
    }
}

and then use one of the fetch methods in JPAQuery:

// fetch first page of results, 25 results per page
post.getComments().fetch(1,25);