how to DRY: ajax action +initial page load

I must be missing something, because I don't get a clean way how to do this.

I have a page which when you change the contents of a select, changes the contents of a div by calling an action.

How do I have the contents of the div be the results of calling the action on the initial page load? I understand if I wanted to render just the template I could use partials, but I also need to have the action function called.

NOTE: the main action should not depend on whether the updatable piece is included on the page.

ie,

controller:

def main
  ...stuff...
  # should not care if page has a change_piece on it, so should not need to do change_piece stuff
end

def change_piece
  ...stuff that must happen...
  when called by ajax:
    render :update do |page|
      page.replace_html :bob, :partial => "piece"
          end
         when called as part of initial page load:
    render :partial => "piece"
end

view (main):

<html>
...stuff...
<div id="bob"> <%= render :action "change_piece" %></bob>
...
</html>

I tried using render_component, with a respond_to block in the action, but it just renders the first thing in the respond_to block. IE, if I have format.html first it renders that first. If I have format.js first, it renders that. (the respond_to block works correctly otherwise)

Thanks,
Michael Johnston