我可以创建一个实体，但我可以'；t编辑一个

这是我的控制器代码：

[HttpPost]
public ActionResult Edit(ProductViewModel viewModel)
{
    Product product = _ProductsRepository.GetProduct(viewModel.ProductId);
    TryUpdateModel(product);
    if (ModelState.IsValid)
    {
        _productsRepository.SaveProduct(product);
        TempData["message"] = product.Name + " has been saved.";
        return RedirectToAction("Index");
    }
    return View(viewModel);     // validation error, so redisplay same view
}

[HttpPost]
public ActionResult Create(CommodityCategoryViewModel viewModel)
{
    Product product = new Product();
    TryUpdateModel(product);
    if (ModelState.IsValid)
    {
        _productsRepository.SaveProduct(product);
        TempData["message"] = product.Name + " has been saved.";
        return RedirectToAction("Index");
    }
    return View(viewModel);     // validation error, so redisplay same view
}

它们都调用Save（）函数，定义如下：

public class ProductsRepository
{
    private readonly MyDBEntities _entities;
    public ProductsRepository()
    {            
        _entities = new MyDBEntities();
    }
    public void SaveProduct(Product product)
    {
        // If it's a new product, just attach it to the DataContext
        if (product.ProductID == 0)
            _entities.Products.Context.AddObject("Products", product);
        else if (product.EntityState == EntityState.Detached)
        {
            // We're updating an existing product, but it's not attached to this data context, so attach it and detect the changes
            _entities.Products.Context.Attach(product);
            _entities.Products.Context.Refresh(System.Data.Objects.RefreshMode.ClientWins, product);
        }
        _entities.Products.Context.SaveChanges();  // Edit function hits here
    }
}

当我调用Create时，它会命中SaveProduct（）函数中的AddObject（），并正确保存产品。

当我调用Edit时，它只在SaveProduct（）函数中命中_enties.Products.Context.SaveChanges（），并且不保存产品。

我做错了什么？

// We're updating an existing product, but it's not attached to this data context, so attach it and detect the changes            
_entities.Products.Context.Attach(product);            
_entities.Products.Context.Refresh(System.Data.Objects.RefreshMode.ClientWins, product);     

Product origProduct =_entities.Products.Where(m=>m.ProductId == product.ProductId).Single();
_entities.Products.Attach(product, origProduct);