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WARNING: The first draft of this page was done by a total n00b. There wasn't any help here for Recaptcha before, so he hopes this does help some people, but other more experienced should clean this up at a later date.

  1. Get your keys from Google/Recaptcha
  2. Install the Recaptcha gem found here (note: for Rails 3 do gem 'recaptcha', :require => 'recaptcha/rails')
  3. Add <%= recaptcha_tags %> on your New Registration view (you must have generated Devise views) the line before your submit button.
  4. Create/Update your RegistrationsController - see the "controllers" section in the README to understand how to set up your own devise controllers, and don't forget to update the routes once you do. Remember that "devise" expects you to use flash[:notice] and flash[:alert] and not flash[:error]. Include the following (the first is for a clean Devise install, and the second is for if you use OmniAuth like me):

Clean

    class RegistrationsController < Devise::RegistrationsController
      def create
        if verify_recaptcha
          super
        else
          build_resource(sign_up_params)
          clean_up_passwords(resource)
          flash.now[:alert] = "There was an error with the recaptcha code below. Please re-enter the code."      
          flash.delete :recaptcha_error
          render :new
        end
      end
    end
  1. In Rails 3.x you may need to add:
# ./config/application.rb
require 'net/http'

to support the Recaptcha gem.

  1. With Ruby 1.9.x, you need to add:
# ./config/routes.rb
devise_for :users, controllers: { registrations: 'registrations' }

With Ruby 1.8.x:

# ./config/routes.rb
devise_for :users, :controllers => { :registrations => 'registrations' }

OmniAuth:

  class RegistrationsController < Devise::RegistrationsController

    def create
      if session[:omniauth] == nil #OmniAuth
        if verify_recaptcha
          super
          session[:omniauth] = nil unless @user.new_record? #OmniAuth
        else
          build_resource
          clean_up_passwords(resource)
          flash[:alert] = "There was an error with the recaptcha code below. Please re-enter the code."
          #use render :new for 2.x version of devise
          render_with_scope :new 
        end
      else
        super
        session[:omniauth] = nil unless @user.new_record? #OmniAuth
      end
    end

Alternate implementation

Warning: I'm not sure I'm any more qualified on this than the OP, but I preferred this setup in my own app.

**Note (advice by reader): If the sign-up is by email as primary key, giving an error that the email is already taken (regardless of recaptcha result) means that it could be used for email harvesting. **

The benefit to this method is that we can add any recaptcha errors to the rest of the form errors instead of an either/or scenario as would happen with the controllers listed above.

Steps 1 - 4 are the same

class Users::RegistrationsController < Devise::RegistrationsController
  def create
    if !verify_recaptcha
      flash.delete :recaptcha_error
      build_resource
      resource.valid?
      resource.errors.add(:base, "There was an error with the recaptcha code below. Please re-enter the code.")
      clean_up_passwords(resource)
      respond_with_navigational(resource) { render_with_scope :new }
    else
      flash.delete :recaptcha_error
      super
    end
  end

  def clean_up_passwords(*args)
    # Delete or comment out this method to prevent the password fields from 
    # repopulating after a failed registration
  end
end

The flash.delete :recaptcha_error gets rid of the default error from the recaptcha gem, which in my case kept appearing as "incorrect-captcha-sol".

For Devise 2.x change

respond_with_navigational(resource) { render_with_scope :new }

into

respond_with_navigational(resource) { render :new }

Notice! Captcha for Login

If you want use captcha on your login page, you need to care about a security issue!

By default, Devise has use require_no_authentication method as before_filter for Session#new action.

That means Devise will auto check login and do login event before you Session#new action, so you verify_recaptcha check will can't work.

So you need skip that method like this:

class SessionsController < Devise::SessionsController
  skip_before_filter :require_no_authentication, :only => [:new]  

  def create
    if verify_recaptcha
      super
    else
      build_resource
      flash[:error] = "Captcha has wrong, try a again."
      respond_with_navigational(resource) { render :new }
    end    
  end
end

More info about this: #2016

Captcha with Devise 2.x

If the above method doesn't work then try

  before_filter :check_user_validation, :only=>:create

  def check_user_validation
     #validate user or redirect with this method
  end
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