...Visitor ..{

........
     Session lastSession;    
........
    /**
     * @hibernate.set table = "Session" cascade="save-update" lazy="true" sort="natural"
     * @hibernate.collection-key column="VisitorID"
     * @hibernate.one-to-many class="Session"
     * @hibernate.cache usage="nonstrict-read-write"
     */
     public  SortedSet<Session> getSessions() {
         return  sessions;
    }
........
}

...Session .. {
   private  Visitor visitor;

..........
    /**
     * @hibernate.many-to-one column="VisitorID"
     */
     public  Visitor getVisitor() {
         return  visitor;
    }

}