can't reassign a type asp.net

I have an Edit scaffold that isn't properly changing a field of mine..

Here's the model for the class I'm trying to edit:

namespace SmartLivingAdvisor.Models
{
    public class SubCategoryClass
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public int? CategoryID { get; set; }
        public virtual CategoryClass CatergoryClass { get; set; }

    }
}

My edit form is not properly reassgning a new CatergoryClass!

Here's the post Edit function:

        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Edit([Bind(Include = "ID,Name,CategoryID")] SubCategoryClass subCategoryClass)
        {
            if (ModelState.IsValid)
            {
                subCategoryClass.CatergoryClass = db.CategoryClasses.Find(subCategoryClass.CategoryID);     
                db.Entry(subCategoryClass).State = EntityState.Modified;
                db.SaveChanges();
                return RedirectToAction("Index");
            }

            return View(subCategoryClass);
        }

This line:

subCategoryClass.CatergoryClass = db.CategoryClasses.Find(subCategoryClass.CategoryID);

Is not doing what it should do. It should assign the new CatergoryClass.