public void AddOrderDetail(OrderDetail od)
        {
            /*
             * using (var db = new OrderContext())
             * {
             *  db.goods.Add(orderItem);
             *  db.SaveChanges();
             * }*/

            if (Sp.Contains(od))
            {
                throw new ApplicationException($"orderDetails-{od} is already existed!");
            }
            Sp.Add(od);
        }