form select box from association

Hello there,

I have the following models

class Bibliography < ActiveRecord::Base
  has_and_belongs_to_many :gthemes, :join_table =>

class Gtheme < ActiveRecord::Base
  has_and_belongs_to_many :bibliographies, :join_table =>

I want to include in the bibliography edit page a select box with all
the gthemes and i want the members of @bibliography.gthemes to be
marked as selected

I 'm using this in bibliography/_form.rhtml:

<%= select('bibliography', 'gthemes' , Gtheme.find(:all).collect{ |gt|
[,]}, {}, {:multiple => true}) %>

but it doesn't mark the current gthemes as selected, any ideas?

Thanks in advance,

I also tried using the form_for helper but the problem still exists

<% form_for :bibliography, @bibliography, :url => { :action =>
"update" } do |f| %>
  Bgthemes: <%= :gthemes, Gtheme.find(:all).collect {|c|
[,]}, {}, {:multiple => true} %>
  <%= submit_tag 'Save changes' %>
<% end %>

For the record, I didn't manage to find a "magic way" to do it, but
this is a nice and working solution:

<%= select_tag 'gthemes[]',
  options_from_collection_for_select(@gthemes, 'id', 'name',
@bibliography.gtheme_ids ),
  {:multiple => true, :class => "wide"}

@gthemes = Gtheme.find(:all)