Rails + Sass + Asset Fingerprints + Heroku

asset_tag_helper.rb

# Ensure AssetTagHelper has been loaded before we try to monkey-patch it.
require 'action_view/helpers/asset_tag_helper'

module ActionView::Helpers::AssetTagHelper
  # Insert the asset id in the filename, rather than in the query string. In
  # addition to looking nicer, this also keeps any other static file handlers
  # from preempting our Rack::StaticCache middleware, since these
  # version-numbered files don't actually exist on disk.
  def rewrite_asset_path(source)
    source.insert source.rindex('.'), "-#{rails_asset_id(source)}"
  end

  # Exactly like the original, with calculate_asset_id extracted.
  def rails_asset_id(source)
    if asset_id = ENV["RAILS_ASSET_ID"]
      asset_id
    else
      if @@cache_asset_timestamps && (asset_id = @@asset_timestamps_cache[source])
        asset_id
      else
        asset_id = calculate_asset_id(source)

        if @@cache_asset_timestamps
          @@asset_timestamps_cache_guard.synchronize do
            @@asset_timestamps_cache[source] = asset_id
          end
        end

        asset_id
      end
    end
  end

  private

  # Check out the sass initializer and the Rack::StaticCache setup in
  # environment.rb. We're serving sass-generated stylesheets from tmp.
  def calculate_asset_id(source)
    case source
    when %r{^/stylesheets}
      digest File.join('tmp', source)
    else
      digest File.join('public', source)
    end
  end

  # Prefer digest of file contents over File mtime, since File mtimes change on
  # every Heroku deploy, even if the contents don't.
  #
  # We additionally to_i(16) the digest since Rack::StaticCache assumes version
  # numbers have digits and dots only.
  def digest(path)
    Digest::MD5.file(path).hexdigest.to_i(16)
  end
end

environment.rb

RAILS_GEM_VERSION = '2.3.8' unless defined? RAILS_GEM_VERSION

require File.join(File.dirname(__FILE__), 'boot')

Rails::Initializer.run do |config|
  # Set far-future caching headers for static assets.
  config.middleware.use 'Rack::StaticCache',
    :root => 'public',
    :urls => ['/images', '/javascripts']

  # Serve sass-generated stylesheets on Heroku.
  config.middleware.use 'Rack::StaticCache',
    :root => 'tmp',
    :urls => ['/stylesheets']
end

sass.rb

# Compile sass templates to tmp, for Heroku.
Sass::Plugin.options[:template_location] = Rails.root.join('app', 'stylesheets').to_s
Sass::Plugin.options[:css_location]      = Rails.root.join('tmp', 'stylesheets').to_s

static_assets_middleware.rb

# Heroku's Rack stack includes a Heroku::StaticAssetsMiddleware that (at least
# as of 2010-06-25) blows away any Cache-Control header coming from a
# Rack::File. This would defeat the purpose of our Rack::StaticCache middleware
# setup (see config/environment.rb), so we monkey-patch it here to do nothing.
module Heroku
  class StaticAssetsMiddleware
    def initialize(app)
      @app = app
    end

    def call(env)
      @app.call(env)
    end
  end
end