实体类型“XXX”不是当前上下文的模型的一部分

Question

我在一个表中插入数据，并希望更新另一个表的数据，就像我们在winforms c＃中一样。我希望当有人购买产品时，其数量将在另一张表中更新。但我收到这个错误： “实体类型产品不是当前上下文模型的一部分。”

我的代码如下：

   [HttpGet]
    public ActionResult Purchase()
    {
        if (Session["userId"] != null)
        {
            int a = (int)Session["userId"];
            var maxId = //(from k in mse.Orders where k.user_Id == a select  k.Order_Id).Max(); 
                mse.Orders.Where(r => r.user_Id == a).OrderByDescending(r => r.Order_Id).First();
            ViewBag.oId = maxId.Order_Id;
            var reg = (from abc in mse.Registerations where abc.Register_Id == a select abc).First();
            ViewBag.shipAd = reg.shippingAddress;
            ViewBag.quantity = maxId.Quantity;
            TempData["quantity"] = ViewBag.quantity;
            var prodId = maxId.Product_Id;
            var pId = (from f in mse.Products where f.Product_Id == prodId select f).OrderByDescending(f => f.Product_Id).First();
            ViewBag.price = pId.unitPrice;

        }


        return View();
    }
[HttpPost]
public ActionResult Purchase(Purchase p)
{
if (Session["userId"] != null)
{
p.Register_Id = (int)Session["userId"];
int a = Convert.ToInt16(TempData["productId"]);
int b = Convert.ToInt16(TempData["quantity"]);

var subQuanitty = (from s in mse.Products where s.Product_Id == a select s).First();
int quantity = Convert.ToInt16(subQuanitty.Quantity) - b;
Products prod = new Products();
prod.Quantity = b.ToString();

updateProduct(prod);

if (ModelState.IsValid)
{
mse.Purchases.Add(p);
mse.SaveChanges();

return RedirectToAction("Status");
}
}

return View(p);
}

public void updateProduct(Products prod)
{
mse.Entry(prod).State = EntityState.Modified;
mse.SaveChanges();
}