public class CategoryShort
    {
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
        public virtual int TotalPriceOfAllItems {get; set; }
    }

 public class CategoryShortMap : ClassMap<CategoryShort>
    {
        public CategoryShortMap()
        {
            this.Not.LazyLoad();
            Table("Category");
            Id(x => x.Id);
            Map(x => x.Name);
        }
    }

        public IEnumerable<CategoryShort> GetAllShort()
        {
            using (var session = NHibernateHelper.SessionFactory.OpenSession())
            {
                return session.CreateCriteria<CategoryShort>().AddOrder(Order.Asc("Name")).List<CategoryShort>();
            }
        }

using (var session = NHibernateHelper.SessionFactory.OpenSession())
            {
                var query = session.CreateSQLQuery(@"SELECT ... ");
  var categoriesTmp = query.SetResultTransformer(Transformers.AliasToBean<CategoryTMP>()).List<CategoryTMP>();

 public class CategoryShortMap : ClassMap<CategoryShort>
    {
        public CategoryShortMap()
        {
            this.Not.LazyLoad();
            Table("Category");
            Id(x => x.Id);
            Map(x => x.Name);
            Map(x => x.ItemsCount).Formula("Select Count(*) From KindOfItems i Where i.CategoryId = Id");
        }