当我尝试在Pro ASP.NET MVC3中完成SportStore演示时，为什么SaveChanges（）无法工作？

public void SaveProduct(Product product)
{
    if (product.ProductID == 0)
         {
            context.Products.Add(product);
         }   
     //Oops~~~
     context.SaveChanges();
}

[HttpPost]
public ActionResult Edit(Product product)
{
    if (ModelState.IsValid)
    {
        repository.SaveProduct(product);
        //I can see this msg int the view page. but database never changed.!!
        TempData["message"] = string.Format("{0} has been saved", product.Name);
        return RedirectToAction("Index");
    }
    else
    {
        // there is something wrong with the data values
        return View(product);
    }
}
public void SaveProduct(Product product)
{