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.
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.