collection_select problem

i’am novice at ruby on Rails framework. i’am trying to add a plugin for Redmin wich is a Rails application.
My problem:
i have this code in my html.erb file:
<%= form_tag :action => “sauver” >

< @types.each do |untype| > < end %>
<= untype[:alertetype] > <= collection_select(:tracker, :id, @trackers, :id, :name, options ={:prompt => "choisir un tracker"}) >

<%= submit_tag “valider” %>

i need to recover all values selected. When i use “params[:tracker][:id]” i have just one value posted.
Could you help me please.
Thank you in advance.

pass the :multiple option for the collection_select