Model value lost on postback - asp.net-mvc

Question

I have the following models:
class A
{
// ...some properties
public B InnerField { get; set; }
}
and
class B
{
public int Id { get; set; }
// ..other properties
}
and a page that has a model Class A and inside the page I have a partial view bound to Class B inside a form.
The value of the Id (in the partial view) is set correctly to the model's Id value (different from 0) but when I submit the page the model has the Id value 0. The Id value is not modified in the component or elsewhere.
Page
...other parts of main page
<%using (Html.BeginForm("ModifyHotel", "Hotel",
FormMethod.Post, new { enctype = "multipart/form-data"}))
{%>
<% Html.RenderPartial("~/Views/Shared/ModifyBaseItem.ascx",
new ModifyItemRequestBaseView() { ItemId = Model.Item.Id });%>
<%}%>
...other parts of main page
Partial View
...other parts of partial view
<br/>
Add Photo: <%:Html.FileBoxFor(x => x.PhotoFile, null)%>
<br/>
Add Video: <%:Html.FileBoxFor(x => x.VideoFile, null)%>
<br/>
<input type="submit" value="Submit changes" />
...other parts of partial view
What can I do to keep the value of the inner model when the post is made?
Thanks,

Answer 1

Controller:
public class HomeController : Controller
{
public ActionResult Index()
{
A model = new A() { InnerField = new B() { Id = 5 }};
return View(model);
}
[HttpPost]
public ActionResult Index(B model)
{
//on postback the model should have the value 5 here
return View();
}
}
View:
#model MvcApplication11.Models.A
#using (Html.BeginForm())
{
#Html.Partial("_IndexForm", Model.InnerField)
<input type="submit" />
}
Partial:
#model MvcApplication11.Models.B
#Html.EditorFor(m => m.Id)