在ViewModel中将值从一个模型传递到另一个模型

Question

I am building my first MVC website and I am struggling to pass a value from one model to another in a ViewModel. The two classes are Music and MusicGenre. In Music, there is a field called 'GenreId' which is an int and is related to the 'Id' field in MusicGenres. I want to return the genre name relating to the Id that's passed over.

Here's what I've got so far:

        MusicViewModel vmMusic = new MusicViewModel
        {
            Music = _context.Music.SingleOrDefault(c => c.Id == id),
            MusicGenres = _context.MusicGenres.Where(gi => gi.Id == xxx).ToList()
        };
        return View(vmMusic); 

This all renders nicely minus displaying the right genre. (I replaced xxx with Id but that just uses the id field from Music, not the GenreId field)

So xxx is where I want to pass 'GenreId' but I don't know how. Any help? Thanks

Answer 1

To expand on my initial comment to the question, I'd just do something like:

    var music = _context.Music.SingleOrDefault(c => c.Id == id);
    var musicGenres = _context.MusicGenres.Where(gi => gi.Id == music.GenreId).ToList(); // whatever the genre id property is called.
    MusicViewModel vmMusic = new MusicViewModel
    {
        Music = music,
        MusicGenres = musicGenre
    };
    return View(vmMusic); 

Answer 2

You should separate your view models from your persistence models. View model is designed to display what you want to display to the users. Persistence model might reflect your data storage structures.

Also from your example it looks like you might already have been using ORM like EntityFramework . EntityFramework provides features like navigation properties and lazy loading.

Either way, I strongly disagree accepted answer 's approach, which sends the music and its genre persistence models to the view, when you only need to get the genre name.

Rather, I will approach it this way:

Persistence Models

You might have one-to-may relationship between a genre and an album in your persistence storage:

public class Album
{
    public int AlbumId { get; set; }
    public string Title { get; set; }
    public double Price { get; set; }

    public int GenreId { get; set; }
    public Genre Genre { get; set; }
}

public class Genre
{
    public int GenreId { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }

    public List<Album> Albums { get; set; }
}

But obviously you don't want/need to reveal everything you have in your persistence storage to the user. Hence you create view models and only contain what you want to display.

View Models

Let's say you have a view to display information about the selected album. You might create a view model like this:

public class AlbumViewModel
{
    public int AlbumId { get; set; }
    public string AlbumName { get; set; }
    public string Genre { get; set; }
}

Then in the album controller, you can build the view model by using any ORM, lazy loading, etc:

Controller

If you happen to use EntityFramework and have lazy loading enable, you can fetch genre name via its navigation property:

public ActionResult Details(int id)
{
    var album = _dbContext.Albums.SingleOrDefault(x => x.AlbumId == id);
    if (album == null)
    {
        return HttpNotFound();
    }

    var vm = new AlbumViewModel
    {
        AlbumId = album.AlbumId,
        AlbumName = album.Title,

        // You can get the genre name via Navigation property, and/or lazy
        // loading
        Genre = album.Genre.Name
    };

    return View(vm);
}

Now in a more advanced architecture, the read and write is separated, which is referred as CQRS. For all the reads (ie, displaying information to the user), you can build your view model with data directly from executing plain SQL statement.

CQRS with Dapper

using Dapper;
using System.Data;

...

public class AlbumController : Controller
{
    private readonly IDbConnection _dbConnection;

    public AlbumController(IDbConnection dbConnection)
    {
        _dbConnection = dbConnection;
    }

    public ActionResult Details(int id)
    {
        const string sql = @"
SELECT a.AlbumId, a.Title AS [AlbumName], g.Name AS [Genre]
FROM [Album] a
    JOIN [Genre] g ON a.GenreId = g.GenreId
WHERE a.AlbumId = @albumId;
";

        var vm = _dbConnection.QuerySingleOrDefault<AlbumViewModel>(sql, new {
            albumId = id
        });
        if (vm == null)
        {
           return HttpNotFound();
        }

        return View(vm);
    }
}