dynamic dropdown...

im working on the code in this link…
http://www.pullmonkey.com/2008/3/30/dynamic-select-boxes-ruby-on-rails

i cant able to dynamically select values in second and third dropdown
using onchange event .. any suggestions pls...

index.rhtml is...

<%= javascript_include_tag :defaults %>

<%= collection_select(nil, :genre_id, @genres, :id, :name,
                      {:prompt => "Select a Genre"},
                      {:onchange =>" alert('sd');"}) %>

<br/>
<div id="artists"><%= render :partial => 'artists', :object => @artists
%></div>
<div id="songs"><%= render :partial => 'songs', :object => @songs
%></div>

controller is...

def update_artists
    # updates artists and songs based on genre selected
     @genre = Genre.find(params[:genre_id])
    @artists = @genre.artists
    @songs = @genre.songs

    render :update do |page|
      page.replace_html'artists', :partial => 'artists'
      page.replace_html 'songs', :partial => 'songs', :object =>
songs
    end
  end

if i select values in first drop down i cant get corresponding values
for second as well as for third.. pls help