Models, ViewModels, DTOs in MVC 3 application

问题:

I have a web solution (in VS2010) with two sub-projects:

1. Domain which holds the Model classes (mapped to database tables via Entity Framework) and Services which (besides other stuff) are responsible for CRUD operations

2. WebUI which references the Domain project

For the first pages I've created I have used the Model classes from the Domain project directly as Model in my strongly typed Views because the classes were small and I wanted to display and modify all properties.

Now I have a page which should only work with a small part of all properties of the corresponding Domain Model. I retrieve those properties by using a projection of the query result in my Service class. But I need to project into a type - and here come my questions about the solutions I can think of:

1. I introduce ViewModels which live in the WebUI project and expose IQueryables and the EF data context from the service to the WebUI project. Then I could directly project into those ViewModels.

2. If I don't want to expose IQueryables and the EF data context I put the ViewModel classes in the Domain project, then I can return the ViewModels directly as result of the queries and projections from the Service classes.

3. In addition to the ViewModels in the WebUI project I introduce Data transfer objects which move the data from the queries in the Service classes to the ViewModels.

Solution 1 and 2 look like the same amount of work and I am inclined to prefer solution 2 to keep all the database concerns in a separate project. But somehow it sounds wrong to have View-Models in the Domain project.

Solution 3 sounds like a lot more work since I have more classes to create and to care about the Model-DTO-ViewModel mapping. I also don't understand what would be the difference between the DTOs and the ViewModels. Aren't the ViewModels exactly the collection of the selected properties of my Model class which I want to display? Wouldn't they contain the same members as the DTOs? Why would I want to differentiate between ViewModels and DTO?

Which of these three solutions is preferable and what are the benefits and downsides? Are there other options?

Thank you for feedback in advance!

Edit (because I had perhaps a too long wall of text and have been asked for code)

Example: I have a Customer Entity ...

public class Customer
{
    public int ID { get; set; }
    public string Name { get; set; }
    public City { get; set; }
    // ... and many more properties
}

... and want to create a View which only shows (and perhaps allows to edit) the Name of customers in a list. In a Service class I extract the data I need for the View via a projection:

public class CustomerService
{
    public List<SomeClass1> GetCustomerNameList()
    {
        using (var dbContext = new MyDbContext())
        {
            return dbContext.Customers
                .Select(c => new SomeClass1
                             {
                                 ID = c.ID,
                                 Name = c.Name
                             })
                .ToList();
        }
    }
}

Then there is a CustomerController with an action method. How should this look like?

Either this way (a) ...

public ActionResult Index()
{
    List<SomeClass1> list = _service.GetCustomerNameList();
    return View(list);
}

... or better this way (b):

public ActionResult Index()
{
    List<SomeClass1> list = _service.GetCustomerNameList();

    List<SomeClass2> newList = CreateNewList(list);

    return View(newList);
}

With respect to option 3 above I'd say: SomeClass1 (lives in Domain project) is a DTO and SomeClass2 (lives in WebUI project) is a ViewModel.

I am wondering if it ever makes sense to distinguish the two classes. Why wouldn't I always choose option (a) for the controller action (because it's easier)? Are there reasons to introduce the ViewModel (SomeClass2) in addition to the DTO (SomeClass1)?

回答1:

introduce ViewModels which live in the WebUI project and expose IQueryables and the EF data context from the service to the WebUI project. Then I could directly project into those ViewModels.

The trouble with this is you soon run into problems using EF trying to 'flatten' models. I encountered something similar when I had a CommentViewModel class that looked like this:

public class CommentViewModel
{
    public string Content { get; set; }
    public string DateCreated { get; set; }
}

The following EF4 query projection to the CommentViewModel wouldn't work as the couldn't translate the ToString() method into SQL:

var comments = from c in DbSet where c.PostId == postId 
               select new CommentViewModel() 
               { 
                   Content = c.Content,
                   DateCreated = c.DateCreated.ToShortTimeString() 
               };

Using something like Automapper is a good choice, especially if you have a lot of conversions to make. However, you can also create your own converters that basically convert your domain model to your view model. In my case I created my own extension methods to convert my Comment domain model to my CommentViewModel like this:

public static class ViewModelConverters
{
    public static CommentViewModel ToCommentViewModel(this Comment comment)
    {
        return new CommentViewModel() 
        { 
            Content = comment.Content,
            DateCreated = comment.DateCreated.ToShortDateString() 
        };
    }

    public static IEnumerable<CommentViewModel> ToCommentViewModelList(this IEnumerable<Comment> comments)
    {
        List<CommentViewModel> commentModels = new List<CommentViewModel>(comments.Count());

        foreach (var c in comments)
        {
            commentModels.Add(c.ToCommentViewModel());
        }

        return commentModels;
    }
}

Basically what I do is perform a standard EF query to bring back a domain model and then use the extension methods to convert the results to a view model. For example, the following methods illustrate the usage:

public Comment GetComment(int commentId)
{
    return CommentRepository.GetById(commentId);
}

public CommentViewModel GetCommentViewModel(int commentId)
{
    return CommentRepository.GetById(commentId).ToCommentViewModel();
}

public IEnumerable<Comment> GetCommentsForPost(int postId)
{
    return CommentRepository.GetCommentsForPost(postId);
}

public IEnumerable<CommentViewModel> GetCommentViewModelsForPost(int postId)
{
    return CommentRepository.GetCommentsForPost(postId).ToCommentViewModelList();
}

回答2:

I would solve your problem by using an auto-mapping tool (like AutoMapper) to do the mapping for you. In cases where the mapping is easy (for example if all properties from one class should be mapped to properties with the same name on another class) AutoMapper will be able to do all the hook-up work for you, and you'll have to give a couple of lines of code to note that there should be a map between the two at all.

That way, you can have your entities in Domain, and a couple of view model classes in your WebUI, and somewhere (preferrably in WebUI or a sub namespace of the same) define maps between them. Your view models will in effect be DTOs, but you won't have to worry much about the conversion process between the domain and your DTO classes.

Note: I would strongly recommend against giving your Domain entities straight to the views of your MVC web UI. You don't want EF to "stick around" all the way to the front-end layer, in case you later want to use something other than EF.

回答3:

Talking about Models, ViewModels and DTOs is confusing, personally I don't like to use these terms. I prefer to talk about Domain Entities, Domain Services, Operation Input/Result (aka DTOs). All of these types live in the Domain layer. Operations is the behavior of Entities and Services. Unless you are building a pure CRUD application the presentation layer only deals with Input/Result types, not Entities. You don't need additional ViewModel types, these are the ViewModels (in other words, the Model of the View). The View is there to translate the Operation Results to HTML, but the same Result could be serialized as XML or JSON. What you use as ViewModel is part of the domain, not the presentation layer.