Question

Using hibernate ctiteria I want to select an object and it’s associated oneToMany list of objects. I want to paginate through this list avoiding the dreaded hibernate n+1 select issue

Here’s a working solution which requires 11 trips to the database for 10 parent objects.

Criteria criteria = this.getSession().createCriteria(Mother.class);
criteria.addOrder(Order.asc("title"))
.setMaxResults(details.getMaxRows())
.setFirstResult(details.getStartResult())
.setFetchMode("kittens", FetchMode.SELECT);
List test = criteria.list();

And here’s a solution which executes only one sql statement (hurray) but cannot handle pagination ie the setMaxResults and setFirstResult are incorrect on the parent object Mother (boo)

Criteria criteria = this.getSession().createCriteria(Mother.class);
criteria.addOrder(Order.asc("title"))
.setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)
.setMaxResults(details.getMaxRows())
.setFirstResult(details.getStartResult())
.setFetchMode("kittens", FetchMode.JOIN);
List test = criteria.list();

This seems like such a common requirement but I’ve dug around for a solution with no luck.

Any takers?

Answer 1

Getting it down to 1 query is tough (i.e. I don’t know a portable solution), but getting it down to 2 queries (irrespective of n) is pretty simple:

Criteria criteria = this.getSession().createCriteria(Mother.class);
criteria.addOrder(Order.asc("title"))
    .setMaxResults(details.getMaxRows())
    .setFirstResult(details.getStartResult())
    .setProjection(Projections.id());
List<?> ids = criteria.list();

criteria = getSession().createCriteria(Mother.class)
    .add(Restrictions.in("id", ids))
    .setFetchMode("children", FetchMode.JOIN)
    .setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)

return criteria.list();

For some databases, subselect fetching children might work, too.