AddToCartHandler.cs

public class AddToCartHandler : ICancellableAsyncRequestHandler<AddToCart, Unit>
{
    private readonly MusicStoreContext _dbContext;
    private readonly ILogger<ShoppingCartController> _logger;

    public AddToCartHandler(MusicStoreContext dbContext, ILogger<ShoppingCartController> logger)
    {
        _dbContext = dbContext;
        _logger = logger;
    }

    public async Task<Unit> Handle(AddToCart message, CancellationToken cancellationToken)
    {
        // Retrieve the album from the database
        var addedAlbum = await _dbContext.Albums
            .SingleAsync(album => album.AlbumId == message.AlbumId, cancellationToken);

        // Add it to the shopping cart
        var cart = Models.ShoppingCart.GetCart(_dbContext, message.CartId);

        await cart.AddToCart(addedAlbum);

        await _dbContext.SaveChangesAsync(cancellationToken);
        _logger.LogInformation("Album {albumId} was added to the cart.", addedAlbum.AlbumId);

        return Unit.Value;
    }
}