SelectList, ASP.NET MVC - asp.net-mvc

Question

I'm working with a SelectList, and populating it with the data from a table. I'm trying to bind it to the ID of another object.
EDIT
Updated the Schema to Reflect something I neglected. I have updated it to show the exact names of each item. I think the problem comes in the fact that each Unit has a sheet, and each sheet has a product. (The sheet will hold more information for the product, but a Unit will have a great deal of other information, so I wanted to separate them in order to keep it clear what was what.)
I think something is happening and the "Sheet" isn't being initialized as an object, and when the binding occurs, it doesn't have an object to bind to, since it is one-level deep. Tell me if this makes sense, or if I am just completely off base.
**Unit**
UnitID (PK)
**ProductSheet**
UnitId (FK)(PK)
ItemId (FK)
**Items**
ItemId (PK)
ItemTitle
It just ...isn't working though. I have this code.
DatabaseDataContext db = new DatabaseDataContext();
Unit unit = new Unit();
ViewData["Items"] = new SelectList( db.Items, "Id", "ItemTitle", unit.ProductSheet.ItemId);
But in the postback, the selectList is always null and empty! This is the View code. I'm really getting lost here, I've followed a lot of examples and it still comes up with bunk.
<%= Html.DropDownList("Items") %>

Answer 1

Your view code should read:
<% var selectItems = ViewData["Items"] as SelectList; %>
<%= Html.DropDownList("ProductSheet.ItemId", selectItems) %>

Answer 2

An entire sample of the project is available at this url
These are the action methods.
public ActionResult Create()
{
DatabaseDataContext database = new DatabaseDataContext();
Unit u = new Unit();
u.Sheet = new Sheet();
ViewData["ProductListing"] = new SelectList(database.Products, "ProductId", "ProductName", u.Sheet.ProductId);
return View();
}
//
// POST: /Home/Create
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Create(Unit u)
{
try
{
DatabaseDataContext database = new DatabaseDataContext();
database.Units.InsertOnSubmit(u);
database.SubmitChanges();
// TODO: Add insert logic here
return RedirectToAction("Index");
}
catch
{
return View();
}
}
Here is the Create View.
<%# Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<MVC.Models.Contexts.Unit>" %>
<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
Create
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<h2>Create</h2>
<%= Html.ValidationSummary("Create was unsuccessful. Please correct the errors and try again.") %>
<% using (Html.BeginForm()) {%>
<fieldset>
<legend>Fields</legend>
<%= Html.DropDownList("ProductListing") %>
<p>
<input type="submit" value="Create" />
</p>
</fieldset>
<% } %>
<div>
<%=Html.ActionLink("Back to List", "Index") %>
</div>
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="ScriptContent" runat="server">
</asp:Content>

Answer 3

I've marked the answer that seems to have made some progress now, I think I'm starting to understand it better.
When I passed the 'path' of the object to assign as a string in the name field of DropDownList, it seems to work. But this doesn't really make a lot of sense to me. Can you explain what is going on any better?