Awesome nested set - how to put there items?

Rails
1

hello guys,

I have following structure of the table, where I store items of tree:

| id         | int(11)  | NO   | PRI | NULL    | auto_increment |
| user_id    | int(11)  | YES  |     | NULL    |                |
| parent_id  | int(11)  | YES  |     | NULL    |                |
| lft        | int(11)  | YES  |     | NULL    |                |
| rgt        | int(11)  | YES  |     | NULL    |                |
| created_at | datetime | YES  |     | NULL    |                |
| updated_at | datetime | YES  |     | NULL    |                |
+------------+----------+------+-----+---------+----------------+

In the form where I create a new items to the tree structure I have in a
hidden input always the parent of item, that I just create.

In a controller I do this:

def create
    @user = User.new(params[:user])

    respond_to do |format|
      if @user.save
        save_tree = TreeStruct.create!(:user_id => @user.id)
        save_tree.move_to_child_of(params[:parent])

        format.html { redirect_to(root_url, :notice => 'OK.') }
        format.xml  { render :xml => @user, :status => :created,

:location => @user }
else
format.html { render :action => “new” }
format.xml { render :xml => @user.errors, :status =>
:unprocessable_entity }
end
end
end

But always when I try to create and item to this tree, I will get the
error:

Couldn't find TreeStruct with id=28

The number 28 is the ID od parent item. The currently created item
saved to database, bud the column parent_id has the value NULL.

I am a bit confusing of it - what I am doing wrong? Why I am getting
this error? And how I should create a root of the tree and then add
other items?