How to add a Rails routes helper (tightly-coupled version).

mapper.rb

# config/initializers/extensions/action_dispatch/routing/mapper.rb
module ActionDispatch
  module Routing
    class Mapper
      module Resources
        alias_method :old_add_route, :add_route

        def add_route(action, options)
          if @scope[:throttle]
            throttlers = @scope.delete(:throttle)

            throttlers.each do |throttler|
              throttle_options = options.dup
              throttle_options[:constraints] = throttler.method(:matches?).to_proc
              throttle_options[:to] = throttler.method(:call).to_proc

              old_add_route(action, throttle_options)
            end
          end

          old_add_route(action, options)
        end

      end

      module Scoping
        def throttle(throttler)
          scope(:throttle => throttler) { yield }
        end

        private

        def merge_throttle_scope(parent, child)
          merged = parent ? parent.dup : []
          merged << child if child
          merged
        end
      end
    end
  end
end

routes.rb

# config/routes.rb
Your::Application.routes.draw do

  class IPThrottler
    class << self
      @cache = {}
      
      attr_reader :cache
    end
    
    def initialize(ttl = 1.seconds)
      @ttl = ttl
    end
    
    def matches?(request)
      cache[request.ip] ||= Time.now
      
      # If @ttl time hasn't passed since the last request, throttle it.
      Time.now - cache[request.ip] > @ttl
    end
    
    def call(env)
      [429, {}, "Too Many Requests"]
    end
    
    def cache
      self.class.cache
    end
  end

  # Return 429 status code if 2 seconds haven't passed since the last request (per IP)
  throttle IPThrottler.new(2.seconds) do
    get "/expensive", "application#expensive_action"
  end

end