Simple Question Based Captcha

model.rb

class User < ActiveRecord::Base

  attr_accessor :captcha_answer
  attr_writer :captcha_question

  def captcha_question
    @captcha_question ||= captcha_questions.to_a[rand(a.length)].first
  end

  def captcha_questions
    { 'What is one plus one?' => ['two', '2'],
      'What number comes after 8?' => ['nine', '9'] }
  end

  validates :check_captcha

  private

  def check_captcha
    captcha_questions[captcha_question].include?(captcha_answer.to_s)
  end
end

view.html.erb

<%= f.hidden_field :captcha_question %>

<%= f.label :captcha_answer, 'Are you human?' %>
<%= f.text_field :captcha_answer %>

Really simple captcha implementation for Active Record models, this has been implemented as a gem, raptcha