Validation of Viewmodel containing nested domain objects - asp.net-mvc

Question

I have the following 2 entities:
public class Product
{
[Key]
public int ID { get; set; }
[Required]
public string Name { get; set; }
public virtual Category Category { get; set; }
}
public class Category
{
[Key]
public int ID { get; set; }
[Required]
public string Name { get; set; }
public ICollection<Product> Products { get; set; }
}
and a view model
public class ProductCreateOrEditViewModel
{
public Product Product { get; set; }
public IEnumerable<Category> Categories { get; set; }
}
The create view for Product uses this ViewModel. The category ID is set as follows in the view:
<div class="editor-field">
#Html.DropDownListFor(model => model.Product.Category.ID,new SelectList
(Model.Categories,"ID","Name"))
#Html.ValidationMessageFor(model => model.Product.Category.ID)
</div>
When the form posts I get an instance of the view model with a product and the selected category object set but since the "Name" property of Category has a [Required] attribute the ModelState is not valid.
As far as creating a Product goes I don't need or care for the "Name" property. How can I get model binding to work such that this is not reported as a ModelState error?

Answer 1

You should create a correct ViewModel for your View.
The best approach imo is not to expose your domain entities to the view.
You should do a simple DTO flattening from your entities to your viewmodel.
A class like that
public class ProductViewModel
{
public int ID { get; set; }
[Required]
public string Name { get; set; }
public int CategoryId? { get; set; }
public SelectList Categories { get; set; }
}
From your controller you map the product to your viewmodel
public ViewResult MyAction(int id)
{
Product model = repository.Get(id);
//check if not null etc. etc.
var viewModel = new ProductViewModel();
viewModel.Name = model.Name;
viewModel.CategoryId = model.Category.Id;
viewModel.Categories = new SelectList(categoriesRepo.GetAll(), "Id", "Name", viewModel.CategoryId)
return View(viewModel);
}
Then in the action that respond to the post, you map back your viewModel to the product
[HttpPost]
public ViewResult MyAction(ProductViewModel viewModel)
{
//do the inverse mapping and save the product
}
I hope you get the idea