Solid approach to loading reference data into view models in ASP.NET MVC

I want a way to separate the loading of reference data into a view model from the controller. At the moment I have a view model with a property for the selected value and the reference data:

public IEnumerable<SelectListItem> DayTypes { get; set; }
public int DayTypeId { get; set; }

and the data is populated from the relevant repository in the controller action:

model.DayTypes = _dayTypeRepository.GetAll().ToSelectList(d => d.Description, d => d.Identifier.ToString());

I would like to change this because it pollutes the controller with lots of repositories and code that is not core to its concerns. All of these dependencies make unit testing the controller a pain.

One possible approach to solving this would be to make the view model class do the loading which would require a custom model binder to instantiate them using the IoC container to provide the repository dependency. Is this a good option?

Another approach that I think would be good is hinted at in CodeCampServer but is incomplete and commented out involving attributes on the field in the view model:

[SelectListProvided(typeof(AllDaysSelectListProvider))]
public IEnumerable<SelectListItem> DayTypes { get; set; }

however I am struggling to figure out how this could be implemented in a way that would not require some major replumbing of the MVC framework.

How do you solve this problem?

EDIT: I want to keep with strongly typed views and avoid stuffing the data into view data.

FURTHER EDIT: I would开发者_StackOverflow also like a solution that is ideally model independent, by which I mean if the same reference data is needed by multiple view models this can be achieved with a single piece of code. Matt's approach is interesting but is tightly coupled to the view model.

---

I would use a service layer which would return me a POCO object that I would map to a view model. So my controller action would look like this:

public ActionResult Index(int id)
{
    var model = _service.GetModel(id);
    var viewModel = Mapper.Map<Model, ViewModel>(model);
    return View();
}

I also like using action filters to avoid the mapping code all over again so:

[AutoMap(typeof(Model), typeof(ViewModel))]
public ActionResult Index(int id)
{
    var model = _service.GetModel(id);
    return View(model);
}

This way only the service talks with the CRUD repositories and the controller talks to the service and the mapping layer.

---

You could write a new ActionFilter that you can decorate an action method with; this action filter will load the reference data into the viewdata, which you can access from your view.

There is more on action filters here.

---

EDIT: Based on the users comments, this now includes a strongly typed option.

Firstly, you need to create the SharedViewModel to contain the shared data.

public class SharedViewModel
{
  public List<string> Days { get; set; }
  public List<string> Months { get; set; }
  public List<string> Years { get; set; }
}

Next, we create the view model to be used by the Index view, which uses this shared view model.

public class HomeViewModel
{
  public string ViewName { get; set; }
  public SharedViewModel SharedViewModel { get; set; }
}

The next step is important, it implements an action filter called SharedData(), which will apply the shared data.

public class SharedDataActionFilter : ActionFilterAttribute
{
  public override void OnActionExecuted(ActionExecutedContext filterContext)
  {
    var currentModel = ((HomeViewModel) filterContext.Controller.ViewData.Model);

    currentModel.SharedViewModel = new SharedViewModel
    {
      Days = new List<string> {"Mon"},
      Months = new List<string> {"Jan"},
      Years = new List<string> {"2011"}
    };

    base.OnActionExecuted(filterContext);
  }
}

At the moment, it just applies the whole shared data, but you can added parameters into the method to be selective.

When the action has been executed, this method takes the current model and adds the shared data.

Here is the controller action.

[SharedDataActionFilter]
public ActionResult Index()
{
  return View("Index", new HomeViewModel { ViewName = "HomePage" });
}

You can access the data like any other strongly typed view, and the shared data wont affect the data already in the model (in this case "ViewName"). You can also use action filters across controllers, and globally across the site with mvc 3.

Hope this helps, Matt.