select_tag question

rubyonrails-talk
1

How can I use select_tag to

1. Have a prompt like "Select a Value" ( selected by default )

I tried this but did not work:

  <%= select_tag :current_course,                  options_for_select( Course.find(:all).map {|p| [p.title,p.id]} ),          { :prompt => "Select a Value" }     %>

2. Generate a 'selected' event so that some action can be done like submit the form or refresh the page?

Thanks in advance - AJ