From http://stackoverflow.com/questions/9587934/how-to-translate-where-not-in-into-an-nhibernate-query-where-the-subquery-tabl

nhibernate_translate_where_not_in_into_query

this.session.Query<Notice>().Where( n => 
    this.session.Query<User>().Any(u => u.UserId == "007"
            && !u.DismissedNotices.Contains(n) );