original: https://github.com/nov/fb_graph_sample/blob/master/lib/authentication.rb / current_fb instead of current_user

authentication.rb

module Authentication

  class Unauthorized < StandardError; end

  def self.included(base)
    base.send(:include, Authentication::HelperMethods)
    base.send(:include, Authentication::ControllerMethods)

    ActiveSupport.on_load(:action_controller) do
      helper_method :current_fb
    end
  end

  module HelperMethods

    def current_fb
      @current_fb ||= Facebook.find(session[:current_fb])
    rescue ActiveRecord::RecordNotFound
      nil
    end

    def authenticated?
      !current_fb.blank?
    end

  end

  module ControllerMethods

    def require_authentication
      authenticate Facebook.find_by_id(session[:current_fb])
    rescue Unauthorized => e
      redirect_to root_url and return false
    end

    def authenticate(user)
      raise Unauthorized unless user
      session[:current_fb] = user.id
    end

    def unauthenticate
      current_fb.destroy
      @current_fb = session[:current_fb] = nil
    end

  end

end