example.rb

# Fun fact... 

redirect_to some_path, notice: 'My notice'

redirect_to some_path, error: 'My error message'

# first will set a flash[:notice] while second will NOT set a flash[:error]

# vs:
flash[:notice] = 'My notice'
redirect_to some_path

flash[:error] = 'My error message'
redirect_to some_path

# Both will set the appropriate flash message. The reason why the difference? 
# ActionController::Flash only handles alert and notice.

flash.rb

module ActionController #:nodoc:
  module Flash
    extend ActiveSupport::Concern

    included do
      delegate :flash, :to => :request
      delegate :alert, :notice, :to => "request.flash"
      helper_method :alert, :notice
    end

    protected
      def redirect_to(options = {}, response_status_and_flash = {}) #:doc:
        if alert = response_status_and_flash.delete(:alert)
          flash[:alert] = alert
        end

        if notice = response_status_and_flash.delete(:notice)
          flash[:notice] = notice
        end

        if other_flashes = response_status_and_flash.delete(:flash)
          flash.update(other_flashes)
        end

        super(options, response_status_and_flash)
      end
  end
end