ASP.NET MVC Done Right: View Models

Question

I read this q/a Real example of TryUpdateModel, ASP .NET MVC 3 and was really interested on @ben-foster response.

I started doing a comment on that answer but got quite long, so started a new Question.

Having ViewModels for everything approach (which i like a lot) get me into some 'weird scenarios' that i want advice in how should I do.

Imagine this structure :

public class ProductListEditableViewModel {
    List<ProductEditViewModel> products {get;set;}
}
public class ProductEditViewModel {
    List<PriceViewModel> prices {get;set;}
}

public class PriceViewModel {
    CurrencyViewModel currency {get;set;}
}

and so on ... ? do you really make one view model for each inner class? how then you map all that to the Model Object?

Also, that covers the Edit, but I have an Add, a send via email, and potentially more Views so more ViewModels!! should i end like something :

AddCurrencyViewModel QuickAddCurrencyViewModel EditCurrencyViewModel ListCurrencyViewModel DeleteCurrencyViewModel ShareCurrencyViewModel

all having the 'almost same' properties ?

Should all those be packed into one file ?

Also do i need all this all viewModels or a inheritance approach might be better?

If you can, I´ll appreciate elaborate on complex scenarios

Also, I use a DTO approach to expose some of the model objects into web service / apis, so I already have some form of mapping already in place where this DTO are not exactly my ViewModels, should I remove one of them? what´s the suggestion in this scenario ?

I´m using entity framework but i think the question is (or should be) ORM agnostic. Not using UoW pattern (will this helps?) as looks it´s gets more complicated as the depth of the object increases.

Thanks a lot!

Answer 1

We typically have a view model per view so yes, if you have lots of views you will have lots of view models.

In typical CRUD applications we often have very similar views, for example Add and Update. In these cases, yes we use inheritance rather than writing duplicate code - usually Add subclasses Update.

public class AddFoo : UpdateFoo {
    public AddFoo() {
        // set up defaults for new Foo
    }
}

public class UpdateFoo {
    public string Name { get; set; }
    // etc.
}

We attempted to "share" view models between views in the past and normally ended up in a world of pain.

With regard to your "weird scenario" - this does look weird indeed, but perhaps because I don't understand your application.

The goal of your view model is to provide the information to the view that is needed and ideally to flatten any complex objects so they are easier to work with. You shouldn't split your view models up like your example unless it makes sense to do so.

Let's say I wanted to a create a view where the customer could change their contact details. Taking the following domain object:

public class Customer {
    public string FirstName { get; set; }
    public string LastName { get;set; }
    public Address Address { get; set; }
}

I'd probably flatten this to a view model like so:

public class UpdateAddressModel {
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string AddressLine1 { get; set; }
    public string AddressLine2 { get; set; }
    public string AddressCity { get; set; }
    // etc.
}

Of course there will be occasions where it doesn't make sense to do this, for example a dashboard view in an online store where you have a list of products going out of stock and a list of recent orders - these two things are unrelated but are required by your view:

public class DashboardModel {
    public List<Product> ProductsGoingOutOfStock { get; set; }
    public List<Order> NewOrders { get; set; }
}

how then you map all that to the Model Object?

I'm assuming by Model Object you mean your data/domain model. The key takeaway here is that the view model you use to render your view is unlikely to be the same as the "models" you POST to the server and if they are, you're probably over-POSTing or you have some crazy enter-everything data capture screen that will make your eyes bleed.

I find it helps to think of what you send to your server as Commands and what you use to render your views as view models.

So the answer to your question - how do you map your complex view model to your data model? - Quite simply, you don't. You should send commands to the server that perform a specific task e.g. updating an address.

There's no hard and fast rule in how you structure your view models but generally go with what makes sense and if it starts to feel too complicated you're probably trying to do too much with one view.

I hope this helps. You'll find lots of posts relating to this matter on my blog.

Answer 2

I realize this is an old-ish question but I did want to address one of the questions posed by the OP that was not answered.

Should all those [ViewModels] be packed into one file ?

Most of the examples I see put each ViewModel in a separate file, so the dominant convention seems to be one file per viewmodel, but I found in practice that this seems to be overkill. Instead I put all viewmodels for a particular controller in one file with multiple viewmodels in it. So for example if User is my Controller and I have several viewmodels associated with this controller such as UserAddViewModel, UserEditViewModel, UserDeleteViewModel I put all of the viewmodels for User in one file called UserViewModels.cs