mvc4 asp.net dropDownList Insert NULL

I search a lot here and find something like this to make dropdown list

this is my controller:
This pass my data for dropDownList...

public ActionResult Create()
  {
      var dba = new WHFMDBContext();
      var query = dba.Categories.Select(c => new { c.Id, c.Name });
      ViewBag.Id = new SelectList(query.AsEnumerable(), "Id", "Name", 3);
      return View();
  }

[HttpPost]
        [InitializeSimpleMembership]
        public ActionResult Create(Profits profits)
        {
            var user = db.UserProfiles.FirstOrDefault(x => x.UserId == WebSecurity.CurrentUserId);
            var profit = new Profits
            {
               Value= profits.Value,
               Description = profits.Description,
               DateInput =profits.DateInput,
               CategoryName =profits.CategoryName,// ???
                User = user,

            };
            db.Profits.Add(profit);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

My View :

@using (Html.BeginForm()) {
    @Html.ValidationSummary(true)

    <fieldset>
        <legend>Profits</legend>
            
        <div class="editor-field">
           @Html.DropDownList("Id", (SelectList) ViewBag.Id, "--Select One--") 
            </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.CategoryName.Id)
            @Html.ValidationMessageFor(model => model.CategoryName.Id)
        </div>

        <div class="editor-label">
            @Html.LabelFor(model => model.Value)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.Value)
            @Html.ValidationMessageFor(model => model.Value)
        </div>

This insert data to database but CategoryName_Id is NULL what am I missing ?
and CategoryName =profits.CategoryName this is a foreign key to Categories in Profits `public Categories CategoryName { get; set; }`

Tags: C#, MVC, ASP.NET