ASP.NET MVC更新数据的有关问题

        public ActionResult Demo()        {            NorthwindEntity entity = new NorthwindEntity();            IQueryable<Products> pro = from s in entity.Products                                       where s.ProductID == 1                                       select s;            return View(pro);        }                //为什么用这个方法不能更新实体类库里的Products数据呢，请问哪里出错啊，请大牛帮忙，谢谢！        public ActionResult UpdateDemo(Products pd)        {            NorthwindEntity entity = new NorthwindEntity();            Products model = entity.Products.FirstOrDefault(e => e.ProductID == pd.ProductID);            UpdateModel(model, pd.ProductName);            entity.SaveChanges();            return RedirectToAction("Demo");        }