in the view:
      <% Candidate::RANKS.each do |rank| -%>
        <div class='form_row'>
          <div class='label'><%= h(rank.titlecase) %></div>
          <div class='item'><%= f.radio_button(:position_rank, rank) %></div>
        </div>

        <%= observe_field("candidate_position_rank_#{rank}",
              :url => {
                :controller => 'faculty_search',
                :action => :rank_radio_button_changed,
                :id => rank},
              :with => "'radio_button_value=' + escape(value)",
              :on => "click") %>
      <% end -%>


in the rjs:

if Candidate::JUNIOR_RANKS.include?(@radio_button_value)
  page << "alert('junior')"
elsif Candidate::SENIOR_RANKS.include?(@radio_button_value)
  page << "alert('senior')"
else
  # no op
end

The problem:
In the browser, the handler only gets fired the first time the thing is clicked.