Models, ViewModels, DTOs in MVC 3 application

Question

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

1. Domain which holds the Model classes (mapped to database tables via Entit

Answer 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 ToCommentViewModelList(this IEnumerable comments)
    {
        List commentModels = new List(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 GetCommentsForPost(int postId)
{
    return CommentRepository.GetCommentsForPost(postId);
}

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