I tried a few hours to solve the following problem: having a class named Group that has zero or more roles, where each role is a String, and the mapping is the following:

<set
    name="roles"
    table="GROUPROLES"
    lazy="true"
    <key column="GROUP"/>
    <element column="ROLE"/>
</set>

create a Criteria based query that returns the groups that has assigned a particular role. Well, this is not posible without resorting to direct SQL, and this is a limitation of Hibernate when one of relationship sides is not a mapped class, like this case that is a String.

The workaround: use a SQL expresion with a susbselect:

Expresion.sql("EXISTS (SELECT * FROM GROUPROLES"
                    + " WHERE GROUP = {alias}.GROUP"
                    + "    AND ROLE = ?)",
              "myrole",
              Hibernate.STRING)