Question

After reading ASP.NET MVC 2 in Action and watching Jimmy Bogard’s presentation from MvcConf (both highly recommended!), I began to implement some of their ideas.

One of the cool things they do, is not only to use AutoMapper to map your entities to some viewmodel, but automate this with an AutoMapViewResult:

public class EventsController : BaseController
{
    public ActionResult Show(Event id) // EntityModelBinder gets Event from repository
    {
        return AutoMapView<EventsShowModel>(id); // AutoMapView<T>(model) is a helper method on the BaseController, that calls AutoMapViewResult<T>(...)
    }
}

// not exactly what you'll find in the book, but it also works :-)
public class AutoMapViewResult<TDestination> : ViewResult
{
    public AutoMapViewResult(string viewName, string masterName, object model)
    {
        ViewName = viewName;
        MasterName = masterName;

        ViewData.Model = Mapper.Map(model, model.GetType(), typeof(TDestination));
    }
}

This all works great, but now there’s a Edit action with its EventsEditModel:

public class EventsEditModel
{
    // ... some properties ...
    public int LocationId { get; set; }
    public IList<SelectListItem> Locations { get; set; }
}

public class EventsController : BaseController
{
    public ActionResult Edit(Event id)
    {
        return AutoMapView<EventsEditModel>(id); 
    }
}

And now (finally) the question:

What do you think, is the best way to get the locations from some sort of data source such as a repository to the EventsEditModel‘s Locations property?

Keep in mind, that I want to use the AutoMapViewResult and a lot of different entity-viewmodel combinations.

Update:

I went with Necros’ idea and created a custom attribute. You can look at the code and download it on my blog ASP.NET MVC: Loading data for select lists into edit model using attributes.

Answer 1

I haven’t gotten to the point (since I saw the talk) when I needed this, but I have a possible solution for this in mind. I think it would work to create an attribute, specifying that this property needs to be loaded. I would start with an abstract class:

public abstract class LoadDataAttribute : Attribute
{
    public Type Type { get; set; }

    protected LoadDataAttribute(Type type)
    {
        Type = type;
    }

    public abstract object LoadData();
}

Then create specific version for each type you want to load (Locations in your case)

public class LoadLocationsAttribute : LoadDataAttribute
{
    public LoadLocationsAttribute() : base(typeof(IList<SelectListItem>))

    public override object LoadData()
    {
        // get locations and return IList<SelectListItem>
    }
}

In your ExecuteResult of AutoMappViewResult you would find all properties with LoadDataAttribute, call LoadData(), cast it to type specified in the attribute and assign it to the property.

I case you just want to load select lists this way, you can just return IList<SelectListItem> instead of object, and save yourself some trouble with casting.

Your view model would the obviously use the attribute.

public class EventsEditModel
{
    // ... some properties ...
    public int LocationId { get; set; }

    [LoadLocations]
    public IList<SelectListItem> Locations { get; set; }
}