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

What you could try is the following: Bind to your ViewModel instead of Product in your post action:

[HttpPost]
public ActionResult Create(ProductEditViewModel viewModel)
{
    if (ModelState.IsValid)
    {
        foreach (var value in viewModel.CategorySelections
                                       .Where(c => c.Selected)
                                       .Select(c => c.Value))
        {
            // Attach "stub" entity only with key to make EF aware that the
            // category already exists in the DB to avoid creating a new category
            var category = new Category { CategoryID = int.Parse(value) };
            db.Categories.Attach(category);

            viewModel.Product.Categories.Add(category);
        }
        db.Products.Add(viewModel.Product);
        db.SaveChanges();
        return RedirectToAction("Index");
    }

    return View(new ProductEditViewModel(
        viewModel.Product, db.Categories.ToList()));
}

I am not sure though if this is the "standard way".

Edit

The return case when the model is invalid cannot work in my example above because viewModel.Product.Categories collection is empty, so you would get no selected category item in the view and not the items which the user had selected before.

I don't know how exactly you bind the collection to the view (your "list of checkboxes"?) but when using a ListBox which allows multiple selection then there seems to be a solution along the lines of this answer: Challenges with selecting values in ListBoxFor. I just had asked Darin in the comments if the list of selected item ids also will get bound to the ViewModel in an post action and he confirmed that.