custom_auth_failure.rb

class CustomAuthenticationFailure < Devise::FailureApp 
  protected 

  def redirect_url 
    "/" 
  end 
end 

devise.rb

# ...
config.warden do |manager|
   manager[:default_strategies][:user].unshift :token_cookie_strategy
end

config.warden do |manager|
  manager.failure_app = CustomAuthenticationFailure
end
# ...

token_cookie_auth_strategy.rb

Warden::Strategies.add(:token_cookie_strategy) do
  def valid?
    token_from_cookie
  end

  def authenticate!
    binding.pry
    if token_from_cookie.present? && (user = User.find_for_token_authentication(:auth_token => token_from_cookie)).present?
      delete_token_from_cookie
      success!(user)
    else
      fail!("Could not log in")
    end
  end

  def token_from_cookie
    cookies[:authentication_token]
  end

  def delete_token_from_cookie
    cookies.delete(:authentication_token)
  end
  
end

users_controller_spec.rb

require 'spec_helper'

describe UsersController do
  before :each do
    @user = FactoryGirl.create(:user)
  end

  describe "User can log in using a token in a signed cookie" do
    it "logs in with token in cookie" do
      @user.ensure_authentication_token!
      cookies[:authentication_token] = @user.authentication_token
      get :personal
      response.should render_template("personal")
      # THE RESPONSE IS REDIRECT
    end
  end
end