Navigation Property not being updated on db.SaveChanges()

Question

I have an Edit controller method that takes a Tool from an Edit view.

The Tool model contains a MobileUser navigation property with name Holder, and it also contains a string HolderName, that's used to set a holder in the view.

In the controller method I set the MobileUser by getting the user with Tool.HolderName == MobileUser.Name, but after db.SaveChanges() is called the MobileUser navigation property is not changes in the database.

My code:

ToolsController.Edit(Tool tool):

        int id = db.MobileUsers.Single(x => x.Name == tool.HolderName).Id;
        tool.Holder = db.MobileUsers.Find(id);

        if (ModelState.IsValid)
        {
            db.Entry(tool).State = EntityState.Modified;
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        return View(tool);

The Holder navigation property in Tool class:

    public virtual MobileUser Holder { get; set; }

Answer 1

I eventually gave up and used GraphDiff instead. Works very well and I recommend it if anyone runs into this problem in the future.