Search according to drop-down list

Rails
1

Hi, currently I have two separate search fields. One searches the
paginated model and the other searches a user model:

<% form_for :post, :method => ‘get’ do %>

<%= collection_select(:post, :id, @posts, :id, :title, options ={:prompt
=> “-Select categories-”}, :class=>“category_dropdown_list”) %>

<%= text_field_tag :search, params[:search] %>
<%= submit_tag “Search”, :name => nil %>
<% end %>

<%= observe_field 'search_form', :frequency => 0.5, :url => { :controller => 'auth_user', :action => 'get_results' }, :loading => "Element.show('indicator_gif_id')", :complete => "Element.hide('indicator_gif_id')", :with => "'search_text=' + escape(value)" %>

I have two main questions:

  1. How would I observe the collection_select and depending on what is
    selected, run x or y action (automatically)
  2. How would I merge the two inputs (I want one input that gets observed
    to act as a ‘live search’)

Thanks!