For legacy schemas only (Hibernate does not generate DDL with triggers).

<id name="id" type="long" column="person_id">
        <generator class="select">
                <param name="key">socialSecurityNumber</param>
        </generator>
</id>

In the above example, there is a unique valued property named socialSecurityNumber defined by the class, as a natural key, and a surrogate key named person_id whose value is generated by a trigger.