GIANT ROBOTS SMASHING INTO OTHER GIANT ROBOTS

Written by thoughtbot

The Accept: header vs. #caches_page

In this example we're going to have an XML API on FontsController#index. A GET to /fonts.xml will produce a list of every Font in the database, along with all its information (name, thumbnail, a list of ligatures, price, license, and so on).

This is a long list. Luckily it's just for the API consumers. The normal HTML people just request /fonts and this gives them a paginated view of the lovely fonts on our system.

So to speed it up we do some simple caching. In FontsController, at the top, we add this:

caches_page :index, :if => Proc.new {|c|
  c.request.format.xml?
}

And magically requests to /fonts.xml are cached to public/fonts.xml. Lovely! Thanks, Rails!

The Problem

So what happens when someone requests /fonts with an Accept: text/xml header? You can try it like this:

curl -H 'Accept: text/xml' http://ihearthelvetica.local/fonts

No XMLFontsController#index uses #respond_to, so it sends back the XML as requested. However, #caches_page saves the XML to public/fonts.html! Now when a user requests /fonts from their Web browser, they're getting back a mess of XML!

No good.

The Workaround

This is a problem deep in the Rails caching code. As a workaround, try this on for size:

class ApplicationController < ActionController::Base
  before_filter :fix_caching_extension_for_xml

private

  def fix_caching_extension_for_xml
    if request.format.xml?
      ActionController::Base.page_cache_extension = '.xml'
    end
  end
end

This manually sets the extension for XML requests to .xml, so that it saves it to the right place.

The Fix

The other option is to fix this in Ruby on Rails itself. Download the patch attached to the ticket I've opened and apply it to an edge version of Rails. The patch has tests and is more generalized, so if the workaround fails to solve your problem the patch might.

Leave a comment on the Lighthouse ticket if the patch works for you, or if you've encountered this problem. Together, we can.