ASP.NET MVC ViewModel with SelectList(s) best practice

Question

I noticed that in the NerdDinner application that if the ModelState is invalid for a dinner, it merely returns the view for the model:

        if (ModelState.IsValid) {
            ...
            return RedirectToAction("Details", new { id=dinner.DinnerID });
        }

        return View(dinner);

However, in my application the model (a view model in this situation) contains multiple SelectLists. These lists are not instantiated at this point because this view model was just populated from the form submission. What is the recommended way to repopulate this SelectLists before sending them back to the user?

This is what I want my controller to do:

public ActionResult Save(MyModel model)
{
    if (ModelState.IsValid)
    {
        businessClass.Save(model);
        return RedirectToAction("Index", "Home");
    }

    // This won't work because model has uninstantiated SelectLists
    return View("MyView", model);
}

I don't want to send the model to my business logic if the ModelState is invalid, but it doesn't seem to make sense to put SelectList population code in my controller. Should I create a public method in my business logic solely for doing this kind of stuff on my view model(s)?

Answer 1

Personally I like to keep it simple:-

[HttpGet]
public Edit(int id) {
     EditForm form = new EditForm();
     // Populate from the db or whatever...
     PopulateEditPageSelectLists(form);
     return View(form);
}

[HttpPost]
public Edit(EditForm form) {
     if (ModelState.IsValid) {
         // Do stuff and redirect...
     }
     PopulateEditPageSelectLists(form);
     return View(form);
}

public void PopulateEditPageSelectLists(form) {
     // Get lookup data from the db or whatever.
}

If the logic to populate the select lists is all kinds crazy it might be worthwhile moving to a separate class or whatever it but as a first step this is the best place to start.