captcha_response.rb

class CaptchaResponse
  RECAPTCHA_API_SERVER        = 'http://api.recaptcha.net'
  RECAPTCHA_API_SECURE_SERVER = 'https://api-secure.recaptcha.net'
  RECAPTCHA_VERIFY_SERVER     = 'api-verify.recaptcha.net'
  attr_writer :user
  attr_reader :errors

  class << self
    private :new
    attr_writer :mode

    def build(ip, challenge, response)
      case @mode
      when :test_valid
        FakeValidResponse.new
      when :test_invalid
        FakeInvalidResponse.new
      else
        new ip, challenge, response
      end
    end
  end

  def initialize(ip, challenge, response)
    @ip = ip
    @challenge = challenge
    @response = response
    @errors = ActiveRecord::Errors.new(self)
  end

  def valid?
    return @valid if instance_variable_defined?(:@valid)

    private_key = ENV['RECAPTCHA_PRIVATE_KEY']
    raise ReCaptchaError, "No private key specified." unless private_key
    begin
      recaptcha = Net::HTTP.post_form URI.parse("http://#{RECAPTCHA_VERIFY_SERVER}/verify"), {
        :privatekey => private_key,
        :remoteip   => @ip,
        :challenge  => @challenge,
        :response   => @response
      }
      answer, error = recaptcha.body.split.map { |s| s.chomp }
      if answer == 'true'
        return @valid = true
      else
        @errors.add_to_base "is incorrect, please try again"
        return @valid = false
      end
    rescue Exception => e
      raise ReCaptchaError, e
    end
  end

  class FakeValidResponse
    def valid?; true end
  end

  class FakeInvalidResponse
    def valid?; false end
  end
end

user.rb

class User < ActiveRecord::Base
  attr_accessor :captcha_response
  validates_presence_of :captcha_response, :on => :create
  validates_associated :captcha_response
end

users_controller.rb

class UsersController < ApplicationController
  resource_controller

  private
  def build_object
    params[:user] ||= { }
    @object ||=
      User.new(params[:user].merge(:captcha_response =>
                                   CaptchaResponse.build(request.remote_ip, params[:recaptcha_challenge_field], params[:recaptcha_response_field])))
  end
end