assetsEntities ass = new assetsEntities();public ActionResult LowValueConsumablesUpdate(tbLowValueConsumables low,string Fid){        tbLowValueConsumables item = ass.tbLowValueConsumables.Include("tbAssetType").FirstOrDefault(p => p.LVid == low.LVid);           item.Amount = low.Amount;           item.TotalPrice = low.TotalPrice           item.tbAssetType.Fid =Fid;;//在這里報錯：屬性“Fid”是對象的鍵信息的一部分，不能修改。            ass.SaveChanges();           return View();}