Question

I bought a book about MVC 3. In this book, there is an example for the implementation of an edit method. Below is the code:

[HttpPost]
public ActionResult Edit(Product product) {
    if (ModelState.IsValid) {
        TryUpdateModel(product); 
        repository.SaveProduct(product);
        return RedirectToAction("Index");
    } else {
        // there is something wrong with the data values
        return View(product);
    }
}

This code works very well for the creation of a newly product but didn’t work for the edition of an exisiting product.

I updated the code for successfully editing a product (see code below):

[HttpPost]
public ActionResult Edit(Product prod)
{
    if (ModelState.IsValid)
    {
        Product product = repository.Products.FirstOrDefault(p => p.ProductID == prod.ProductID);
        TryUpdateModel(product);
        repository.SaveProduct(product);
        return RedirectToAction("Index");
    }
    else
    {
        // there is something wrong with the data values
        return View(prod);
    }
}

EDIT

Here is the Saveproduct method:

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

As you can see, the modification I do in the code is about retrieving the product based on his ID, then save the product.

My question: why the second method works and not the first one? In the second method, why do we have to retrieve the product from the repository?

Thanks.

Answer 1

You don’t need the extra TryUpdateModel method call because MVC has already updated your model when you receive it as a parameter in your Edit method.

[HttpPost]
public ActionResult Edit(Product product) {
    if (ModelState.IsValid) {
        repository.SaveProduct(product);
        return RedirectToAction("Index");
    } else {
        // there is something wrong with the data values
        return View(product);
    }
}

Your SaveProduct method does not handle the editing of a detached entity correctly.

public void SaveProduct(Product product)
{
    if (product.ProductID == 0)
    {
        context.Products.Add(product);
    }
    else
    {
       var entry = context.Entry(product);
       entry.State = EntityState.Modified;
    }         
    context.SaveChanges();
}