How to efficiently update models with references(dropdownList )with asp mvc and nhibernate

Question

Hello, I started to develop a web application (application portfolio) with nhibernate and asp mvc2.

I have some difficulties to properly change the category of an application.
Here are my models:

 public class Application 
 {
    public virtual int  Application_ID{ get; private set; }
    public virtual string Name { get; set; }
    public virtual Category Category { get; set; }
 }
 public class Category : ILookupItem
 {
    public virtual int Category_ID { get; set; }
    public virtual string Name { get;  set; }
 }

My viewModel:

public class ApplicationEditModel
{
    public Application Application { get; set; }
    public SelectList Categories { get; set; }
}

My Form:

<% Html.BeginForm(new {id= Model.Application.Application_ID }); %>
<table>
<tr>
    <td><%=Html.LabelFor(x => x.Application.Application_ID)%></td>
    <td><%=Html.DisplayFor(x=>x.Application.Application_ID) %></td>
</tr>
<tr>
    <td><%=Html.LabelFor(x=>x.Application.Name) %></td>
    <td><%=Html.EditorFor(x=>x.Application.Name) %></td>
</tr>
<tr>
    <td><%=Html.LabelFor(x=>x.Application.Category) %></td>
<td><%=Html.DropDownListFor(x=>x.Application.Category.Category_ID,Model.Categories,"Select a category") %></td>
</tr>
<tr><td><input type="submit" /></td></tr>
</table>
<% Html.EndForm(); %>

My controller action:

    [HttpPost]
    public ActionResult Edit(int id, FormCollection collection)
    {   
        Application app =  _service.FindById(id);
        TryUpdateModel<Application>(app, "Application");
        _service.CommitChanges();
        return RedirectToAction("Index");
    }

I can assign a new category, but if I change to a different category I get the following message:

identifier of an instance of Core.Model.Category was altered from 2 to 3

This seems to be because the defaultmodelbinder is updating the key of the assigned category instead of assigning a new category with new new key.

What is the correct way to update an entity with all its references?
I could maybe use a custom view model, bind it in the controller, and then map it to my domain model. But I fear that it will give me too much work (at the end I will have around 100 properties,30 references and 5-6 lists in my application model).
Could Automapper be useful in this case to update an existing domain model?
How are you handling this kind of update?

Answer 1

It is very hard to use the same model as both view and domain model unless you have a very simple application. Another reason to have a separate domain model is that it is not very user friendly to show a huge form for all of the 100 properties. You can better show the user different forms for different tasks. When you do that, you end up with different viewmodels for one domain entity anyway.