Linq中的group by多表多字段，Sum求和

 var data = (from a in Items
             group a by new { a.GroupId, a.Id } into b
             orderby new ComparerItem() { GroupId = b.Key.GroupId, Id = b.Key.Id } descending
             select new
              {
                 GroupId = b.Key.GroupId,
                 Id = b.Key.Id,
                 Count = b.Sum(c => c.Count),
                 Weight = b.Sum(c => c.Weight)
               }).OrderBy(t => t.GroupId).ThenBy(t => t.Id);