Fetching data within an ASP.NET MVC ViewModel class?

Question

For those that create ViewModels (for use by typed views) in ASP.NET MVC, do you prefer to fetch the data from a service/repository from within the ViewModel, or the controller

Answer 1

There's more than that ;-) You can fetch in model binder or action filter. For the second option, check Jimmy Bogard's blog somewhere around here. I personally do it in model binders. I use ViewModel like this: My custom ASP.NET MVC entity binding: is it a good solution?. It is processed by my custom model binder:

public object BindModel(ControllerContext c, BindingContext b)
{
   var id = b.ValueProvider[b.ModelName]; // don't remember exact syntax
   var repository = ServiceLocator.GetInstance(GetRepositoryType(b.ModelType));
   var obj = repository.Get(id);
   if (obj == null)
     b.ModelState.AddModelError(b.ModelName, "Not found in database");
   return obj;
}

public ActionResult Action(EntityViewModel order)
{
   if (!ModelState.IsValid)
      ...;
}

You can also see an example of model binder doing repository access in S#arp Architecture.

As for static data in view models, I'm still exploring approaches. For example, you can have your view models remember the entities instead of lists, and

public class MyViewModel { public MyViewModel(Order order, IEmployeesSvc _svc) { }

  public IList GetEmployeesList()
  {
      return _svc.GetEmployeesFor(order.Number);
  }

}

You decide how you inject _svc into ViewModel, but it's basically the same as you do for controller. Just beware that ViewModel is also created by MVC via parameterless constructor, so you either use ServiceLocator or extend MVC for ViewModel creation - for example, inside your custom model binder. Or you can use Jimmy Bogard's approach with AutoMapper which does also support IoC containers.

The common approach here is that whenever I see repetative code, I look to eliminate it. 100 controller actions doing domain-viewmodel marshalling plus repository lookup is a bad case. Single model binder doing it in generic way is a good one.