asset-helpers.rb

module Helpers
  def javascript_tag(pathname)
    "<script src='#{asset_url(pathname)}'></script>"
  end

  def stylesheet_tag(pathname)
    "<link href='#{asset_url(pathname)}' rel='stylesheet'>"
  end

  private 
    def asset_url(pathname)
      dir = File.dirname(pathname)
      ext = File.extname(pathname)
      basename = File.basename(pathname, ext)
      time = File.mtime(File.join(PUBLIC_PATH, pathname)).to_i.to_s
      File.join(dir, "#{basename}__#{time}#{ext}")
    end
end

nginx.conf

  rewrite (.*)__([0-9]+)(\.js) $1$3 break;
  rewrite (.*)__([0-9]+)(\.css) $1$3 break;