Hibernate Criteria API - how to order by collection size?

Question

Assuming I have classes User and UserGroup. There is an optional 1-many group to user association and the association is mapped from both sides (the UserGroup side via a pro

Answer 1

It is not possible by default using Citeria API but you can extend org.hibernate.criterion.Order class. This article is about how to extend that class: http://blog.tremend.ro/2008/06/10/how-to-order-by-a-custom-sql-formulaexpression-when-using-hibernate-criteria-api

My sollution is:

public class SizeOrder extends Order {

    protected String propertyName;
    protected boolean ascending;

    protected SizeOrder(String propertyName, boolean ascending) {
        super(propertyName, ascending);
        this.propertyName = propertyName;
        this.ascending = ascending;
    }

    public String toSqlString(Criteria criteria, CriteriaQuery criteriaQuery) throws HibernateException {
        String role = criteriaQuery.getEntityName(criteria, propertyName) + '.' + criteriaQuery.getPropertyName(propertyName);
        QueryableCollection cp = (QueryableCollection) criteriaQuery.getFactory().getCollectionPersister(role);

        String[] fk = cp.getKeyColumnNames();
        String[] pk = ((Loadable) cp.getOwnerEntityPersister())
                .getIdentifierColumnNames();
        return " (select count(*) from " + cp.getTableName() + " where "
                + new ConditionFragment()
                        .setTableAlias(
                                criteriaQuery.getSQLAlias(criteria, propertyName)
                        ).setCondition(pk, fk)
                    .toFragmentString() + ") "
                + (ascending ? "asc" : "desc");
    }

    public static SizeOrder asc(String propertyName) {
        return new SizeOrder(propertyName, true);
    }
    public static SizeOrder desc(String propertyName) {
        return new SizeOrder(propertyName, false);
    }
}

The toSqlString method is based on org.hibernate.criterion.SizeExpression class. (Source: http://javasourcecode.org/html/open-source/hibernate/hibernate-3.6.0.Final/org/hibernate/criterion/SizeExpression.java.html)

Example usage:

hibernateSession.createCriteria(UserGroup.class).addOrder( SizeOrder.asc("members") ).list();

this will list user groups ordered by the size of the members ascending, where "members" is the User collection in UserGroup entity.