authentication.rb

class Authentication < ActiveRecord::Base
  belongs_to :user
  attr_accessible :provider, :uid

  def self.find_by_provider_and_uid(provider, uid)
    where(provider: provider, uid: uid).first
  end
end

error_log

F....

Failures:

  1) OmniauthCallbacksController#guest user facebook email doesn't exist creates authentication using facebook
     Failure/Error: get :facebook
     NoMethodError:
       undefined method `[]' for nil:NilClass
     # ./app/controllers/omniauth_callbacks_controller.rb:4:in `all'
     # ./spec/controllers/omniauth_callbacks_controller_spec.rb:18:in `block (4 levels) in <top (required)>'

Finished in 0.53103 seconds
5 examples, 1 failure

Failed examples:

rspec ./spec/controllers/omniauth_callbacks_controller_spec.rb:17 # OmniauthCallbacksController#guest user facebook email doesn't exist creates authentication using facebook

omniauth_callbacks_controller.rb

class OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def all
    omniauth = request.env["omniauth.auth"]
    authentication  = Authentication.find_by_provider_and_uid(omniauth['provider'], omniauth['uid'])

    if authentication
      flash[:notice] = I18n.t "devise.omniauth_callbacks.success", :kind => omniauth['provider']
      sign_in_and_redirect authentication.user
    else
      user = User.where(:email => omniauth['info']['email']).first
      if user
        user.name = omniauth['info']['name']
        user.authentications.create!(provider: omniauth['provider'], uid: omniauth['uid'])
        flash[:notice] = I18n.t "devise.omniauth_callbacks.success", :kind => omniauth['provider']
        sign_in_and_redirect user
      else
        flash[:alert] = "Registrations are not open yet, but please check back soon"
        redirect_to root_path
      end
    end
  end

  # todo add other provider
  alias_method :facebook, :all
end

omniauth_callbacks_controller_spec.rb

require 'spec_helper'

describe OmniauthCallbacksController do
  include Devise::TestHelpers
  
  before do
    request.env["devise.mapping"] = Devise.mappings[:user]
    request.env["omniauth.auth"] = OmniAuth.config.mock_auth[:facebook]
  end

  describe "#guest user" do
    context "facebook email doesn't exist" do
      before(:each) do
        @user = Fabricate(:user)
        omniauth_mock('facebook', @user.email, @user.name)
      end

      it "creates authentication with facebook" do
        get :facebook
      end
    end
  end
end

spec_helper.rb

OmniAuth.config.test_mode = true
OmniAuth.config.add_mock(:facebook, {
  :provider => "facebook",
  :uid => "1234", 
  :info   => { 
      :name => "Juan Tamad", 
      :email   => "[email protected]" 
  }