How to validate models while associating between two objects - asp.net-mvc

Question

I am building a application using Entity Framework 6.1 code first and ASP.NET MVC5.
I got the following model:
class Person {
public Int32 Id {get; set;}
[Required]
public String Name {get; set;}
public List<Book> Books {get; set;}
}
class Book {
public Int32 Id {get; set;}
[Required]
public String Title {get; set;}
public List<Person> Authors {get; set;}
}
The two basic edit-views for changing Person.Name or Book.Title are straight forward.
Now I got one "real world"-view where the user can edit a Book and add existing Authors to them.
The view posts the values like this:
Title = "Nice Book"
Authors[0].Id = 7
Authors[1].Id = 3
The view will only send the Id of the Persons/Authors. After all, the user won't rename an author while editing a Book.
All the model binding works great and the controller can map the posted fields.
Yet the validation of the bound model fails when the author/Person has no (required) Name set.
Is there some kind of workaround? Currently I omit the RequiredAttribute for the affected properties. I would like to not make a view-model for every aspect of my (rather complex) model-graph where some or another item will get associated.

Answer 1

This is why you should use view models. As they allow you to customize what properties should be exposed to the view. However, you're also no handling the posting of selected authors correctly. For a multiselect like this you really need a view model.
public class BookViewModel
{
public BookViewModel()
{
SelectedAuthorIds = new List<int>();
}
[Required]
public string Title { get; set; }
public List<int> SelectedAuthorIds { get; set; }
public IEnumerable<SelectListItem> AuthorChoices { get; set; }
}
Notice, first, that there's no Id property. You may need to add this if you want to use this same view model to edit books, as well, but for the purposes of creation, there's no need to have an Id field in the view, so we leave it off. Next, there's no Authors property. Instead we have SelectedAuthorIds which is a list of integers and AuthorChoices which will hold all the Authors a user can select from. Finally, there's a constructor to initialize SelectedAuthorIds so it's never null. An empty list translates to nothing being selected, but we need an actual list.
Now, use this view model as the model for your view:
#model Namespace.To.BookViewModel
And, change your author select field(s) to:
#Html.ListBoxFor(m => m.SelectedAuthorIds, Model.AuthorChoices)
You'll also need to populate AuthorChoices, which you'll handle in your GET action for this form:
public ActionResult Create()
{
var model = new BookViewModel();
model.AuthorChoices = db.Authors.Select(m => new SelectListItem
{
Value = m.Id.ToString(),
Text = m.Name
});
return View(model);
}
Finally, after the user posts the form, you'll need to use SelectedAuthorIds to fetch those authors from the database and add them to your Book being created. As far as that goes, you'll also need to map the data from BookViewModel to an instance of Book. All this means is copying over the data from one to the other. There's libraries for this type of thing, such as AutoMapper, but this is relatively simple, so I'll handle it manually:
[HttpPost]
public ActionResult Create(BookViewModel model)
{
if (ModelState.IsValid)
{
var book = new Book
{
Title = model.Title
};
book.Authors = db.Authors.Where(m => model.SelectedAuthorIds.Contains(m.Id));
db.Books.add(book);
db.SaveChanges();
return RedirectToAction("Index");
}
return View(model);
}
The edit version will be similar, except you'll instead pull an existing Book instead of creating a new one:
[HttpPost]
public ActionResult Edit(int id, BookViewModel model)
{
var book = db.Books.Find(id);
book.Title = model.Title;
...
}