MVC4: ViewModel (with radiobuttonlist) is empty after HttpPost - asp.net-mvc-3

Question

I'm having a hard time getting the values from a small multiple choice questionnaire posted to the Controller in my MVC4 app:
The model looks like this:
public class Evaluation
{
public int Id { get; set; }
public IEnumerable<MultipleChoiceQuestion> Question { get; set; }
public Remark Rem { get; set; }
}
public class MultipleChoiceQuestion
{
public int Id { get; set; }
public string Question { get; set; }
public MultipleChoiceAnswer Answer { get; set; }
}
public enum MultipleChoiceAnswer
{
DISAGREE,
NEUTRAL,
AGREE,
NA,
}
This is the View (leaving out some markup):
#model Models.Evaluation
#using (Html.BeginForm("EvaluationB", "Evaluation", FormMethod.Post))
{
#foreach (var item in Model.Question)
{
#Html.DisplayFor(model => item.Question)
#Html.EditorFor(model => item.Question, "Enum_RadioButtonList", new { Id = item.Id })
}
#Html.Label("Remark")
#Html.TextAreaFor(model => model.Rem)
<input type="submit" value="Next" />
}
The "Enum_RadioButtonList" is a View a grabbed from here: https://gist.github.com/973482. It seems like the best way to show enum values in a radiobuttonlist (tho their should be an easier way in MVC 4)
The Controller looks like this:
public ActionResult EvaluationA()
{
Models.Evaluation evm = new Models.Evaluation();
evm.Question = db.MultipleChoiceQuestions.ToList(); //feeding the View some predefined questions
return View(evm);
}
public ActionResult EvaluationB(Models.Evaluation ev)
{
if (ModelState.IsValid)
{
// TODO: save model
return View("EvaluationB", evm);
}
return View("EvaluationA", ev);
}
The questions are loaded fine in the View, but for some reason, the model posted to the Controller remains empty after an HttpPost, and i don't understand why.

Answer 1

I did not see the form have mapping for the Id like below:
#Html.HiddenFor(model => model.Id)
Each of the inner collection of question should also have Id so that it is posted along with the form. So inside the foreach loop in your form for each question you can have:
#Html.HiddenFor(model => item.Question.Id)
Also the model has a collection of type MultipleChoiceQuestion. For model binding to the collection the name of the collection elements should have name attribute with ordered numbers as explained in this post http://haacked.com/archive/2008/10/23/model-binding-to-a-list.aspx

Answer 2

Ive done some more research stumbled upon this post:
http://dotnetslackers.com/articles/aspnet/Understanding-ASP-NET-MVC-Model-Binding.aspx
The problem was in the naming of the html fields. Ive added my View like this and now i can read out the values in the Controller correctly:
#for (int i = 0; i < 6; i++)
{
<tr>
<td>
#Html.DisplayFor(m => m.Question[i].Question)
</td>
<td class="mult_question">
#Html.EditorFor(m => m.Question[i].Answer, "Enum_RadioButtonList" )
</td>
</tr>
}
<tr>
<td>
#Html.Label("Remark")
#Html.TextAreaFor(m => m.Remark)