1:partial.rb

module ApplicationHelper
  def partial(file, opts = {})
    opts[:partial] = file
    controller.__send__ :render_to_string, opts
  end
end

2:controller.rb

class FooController < ApplicationController
  def confirm
    ...
    render :update do |page|
      html = partial("foo")
      logger.debug "created partial content: #{html}"  # easy for debug!
      page[:record].replace_html html
    end
  end
end