Question

I am trying to determine the best way to page/limit the rows returned when querying the children of a OneToMany relationship while using JPA in Play! Framework.

@Entity
public class User extends Model {

    @OneToMany(mappedBy="user", cascade=CascadeType.ALL) 
    public List<CaseFolder> caseFolders;

}


public class CaseFolder extends Model {

    @Required
    @ManyToOne 
    public User user;

    @Required
    public String number;

    public String description;

}

I realize I can set the relationship the lazy fetching. However, that still doesn’t seem to stop me from retrieving the entire list of CaseFolders when I finally access user.caseFolders.

Ideally, I would like to be able to do something like:

user.getCaseFolders().start(100).limit(10)

but I can’t find anything about doing that “out of the box”.

Does everyone really bring the entire related data set into memory every time they need a few of the “children” of a 1-m relationship?

I’m implementing a UI that has paging (using jqgrid) and a user can potentially have thousands of records.

Answer 1

After further research, here’s the answer I came up with:

The “many” side of the relationship – the collection of all caseFolders – is a property of the User. By definition, that property’s value is the entire collection.

To obtain a subset of those elements, I added the following method to my User model:

public List<CaseFolder> getCaseFolders(String sidx, String sord, int start, int limit) {

    String orderBy = "c." + sidx + " " + ((sord.toUpperCase().equals("ASC")) ? "ASC" : "DESC");

    String jpql =   "SELECT c " +
                    "FROM CaseFolder c " +
                    "WHERE c.user = :user " +
                    "ORDER BY " + orderBy;

    Query query = JPA.em().createQuery(jpql)
        .setParameter("user", this)
        .setFirstResult(start)
        .setMaxResults(limit);

    List<CaseFolder> result = query.getResultList();
    return result;

}