How do I ModelBind a many-to-many relationship with MVC 3 and Entity Framework Code First?

Question

I\'m coming across the same problem in my MVC 3 applications. I\'ve got a view to create an new product and that product can be assigned to one or more categories. Here are

Answer 1

Thanks @Slauma, that got me on the right track. Here is my Create and Edit post methods that detail how to manage the relationships (the edit is a bit trickier, because it has to add items that don't exist in the database and delete items that have been removed and do exist in the database). I added a SelectedCategories property (List of ints) to my ProductEditViewModel to hold the result from the form.

[HttpPost]
public ActionResult Create(ProductEditViewModel)
{
    viewModel.Product.Categories = new List();

    foreach (var id in viewModel.SelectedCategories)
    {
        var category = new Category { CategoryID = id };
        db.Category.Attach(category);

        viewModel.Product.Categories.Add(category);
    }

    if (ModelState.IsValid)
    {
        db.Products.Add(viewModel.Product);
        db.SaveChanges();
        return RedirectToAction("Index");
    }

    return View(new ProductEditViewModel(viewModel.Product, GetCategories()));
}

For the Edit method I had to query the database for the current product and then compare that with the viewModel.

[HttpPost]
public ActionResult Edit(ProductEditViewModel viewModel)
{
    var product = db.Products.Find(viewModel.Product.ProductID);

    if (ModelState.IsValid)
    {
        UpdateModel(product, "Product");

        var keys = product.CategoryKeys; // Returns CategoryIDs

        // Add categories not already in database
        foreach (var id in viewModel.SelectedCategorys.Except(keys))
        {
            var category = new Category { CategoryID = id }; // Create a stub
            db.Categorys.Attach(category);

            product.Categories.Add(Category);
        }

        // Delete categories not in viewModel, but in database
        foreach (var id in keys.Except(viewModel.SelectedCategories))
        {
            var category = product.Categories.Where(c => c.CategoryID == id).Single();

            product.Categories.Remove(category);
        }

        db.SaveChanges();
        return RedirectToAction("Index");
    }
    else
    {
        // Update viewModel categories so it keeps users selections
        foreach (var id in viewModel.SelectedCategories)
        {
            var category = new Category { CategoryID = id }; // Create a stub
            db.Categories.Attach(category);

            viewModel.Product.Categories.Add(category);
        }
    }

    return View(new ProductEditViewModel(viewModel.Product, GetCategories()));
}

It is more code that I was hoping it would be, but it is actually pretty efficient with using the stubs and only adding/deleting what has changed.