public class ArtistsByUrlQuery
    {
        private string _url { get; set; }

        public ArtistsByUrlQuery(string url)
        {
            _url = url;
        }

        public ICriteria GetQuery(ISession session)
        {
            var criteria = DetachedCriteria
                .For(typeof(ArtistAlias))
                .Add(Restrictions.Eq("URL", _url))
                .SetProjection(Projections.Property("Artist.Id"));

            return session
                .CreateCriteria(typeof (Artist))
                .Add(Restrictions.Or(Subqueries.PropertyIn("Id", criteria), Restrictions.Eq("URL", _url)));
        }
    }