Facebook real-time updates Rails Metal controller.

application_helper.rb

module ApplicationHelper
  def signature(str, key = nil)
    key ||= CONFIG['facebook']['secret'] if defined?(CONFIG)
    OpenSSL::HMAC.hexdigest(OpenSSL::Digest::Digest.new('sha1'), key, str)
  end
end

facebook_controller.rb

class FacebookController < ActionController::Metal
  include ApplicationHelper

  def notify
    if payload?
      # do something clever with the request body.
      respond_with 200
    else
      respond_with 400
    end
  end

  def verify
    if params['hub.mode'] == 'subscribe' && params['hub.verify_token'] == CONFIG['facebook']['verify']
      respond_with params['hub.challenge'], 200
    else
      respond_with 400
    end
  end

  private

  def body
    @body ||= request.body.read
  end

  def payload?
    request.headers['HTTP_X_HUB_SIGNATURE'] =~ /^sha1=([0-9a-z]{40})$/
    $1 && signature(body) == $1
  end

  def respond_with(body = '', status)
    self.response_body = body
    self.status = status
  end
end

routes.rb

Facebook::Application.routes.draw do
  match 'facebook', to: FacebookController.action(:notify), via: :post, as: :facebook_notify
  match 'facebook', to: FacebookController.action(:verify), via: :get, as: :facebook_verify
end

Stumbled upon this gist and it was very helpful - thanks very much!