dynamic dropdown...

im working on the code in this link... PullMonkey Blog » Blog Archive » 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