MVC 2 viewmodels question

Question

Just working through a tutorial on asp.net/mvc for the music store application.

Ref: http://www.asp.net/mvc/tutorials/mvc-music-store-part-3

In the controller they are passing a list of genres to the view model, I am just a beginner but I feel like it is the viewmodel's job to present the data in what ever format the view requires.

the tutorial code does this...

public ActionResult Index() 
{     
    // Retrieve list of Genres from database     
    var genres = from genre in storeDB.Genres select genre.Name;       

    // Set up our ViewModel     
    var viewModel = new StoreIndexViewModel()
    {         
        Genres = genres.ToList(),         
        NumberOfGenres = genres.Count()
    }; 

    // Return the view     
    return View(viewModel);
}

What I want to do is pass genres to the viewModel and inside the viewModel create the list as well as set the NumberOfGenres property. The way this is coded the controller has to know more about the view than it needs to.

can someone show me what my viewModel class would look like in order to use the ToList() and Count() methods on the genres property inside my viewModel ?

Answer 1

I'm going to disagree with you. The controller knows nothing about the view, only the model. The view model, IMO, should be a simple container just as it is in the tutorial. It's the controller's job to fill up the container with data and pass it off to the view.

It's an open question whether the view model is as simple as it needs to be, i.e. you can easily derive the number of genres from the list of them so it's not really necessary to have it as a separate property. If you wanted to just have the list stored in the model, all you would have to do is invoke the Count() method in the view instead of doing it in the controller.

Not knowing the tutorial, I'm not sure whether they've done it this way in anticipation of adding paging to the model, however. If you did want to support paging in the model, then you would want the total count as a separate property since you'd only be passing part of the collection to the view.

Answer 2

I may be wrong but you seem confused about how viewModel is constructed. The code is doing exactly what you described.

ToList() method returns the query's result that was written above as a List. Mind that it's not executed till ToList() is called.

After getting results from database, they are assigned to the properties in StoreIndexViewModel class.

If we write the code in another way it may be easier to understand

public ActionResult Index() 
{     
    // Retrieve list of Genres from database     
    var genres = from genre in storeDB.Genres select genre.Name;
    var genresList=genres.ToList();       

    // Set up our ViewModel     
    StoreIndexViewModel viewModel = new StoreIndexViewModel()
    viewModel.Genres=genresList;
    viewModel.NumberOfGenres=genresList.Count;

    // Return the view     
    return View(viewModel);
}

Update:

You don't generate the list in the ViewModel. You get them from the database. If you take look at the LINQ expression, it gets the genre names only. Later these results are assigned to the property of the class.

ViewModels are used to provide additional info to the View. Here it's used to show other genres only. It only uses the their names so users can browse them. I didn't check the tutorial but creating,deleting or editing genres need their own functions.