Sample Devise Implementation w/ OmniAuth

app.controllers.omniauth_callbacks_controller.rb

class OmniauthCallbacksController < Devise::OmniauthCallbacksController

  alias_method :facebook, :do_omniauth
  alias_method :google, :do_omniauth

  # It provides central callback for OmniAuth
  def do_omniauth

    @user = User.from_omniauth(request.env['omniauth.auth'])
    provider_kind = @user.provider.capitalize

    if @user.persisted?
      # This will throw if @user is not activated
      sign_in_and_redirect @user, :event => :authentication
      if is_navigational_format?
        set_flash_message(:notice, :success, :kind => provider_kind)
      end
    else
      redirect_to new_user_registration_url
    end

  end

  def failure
    redirect_to root_path
  end

end

app.models.user.rb

class User < ActiveRecord::Base

  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable,
         :omniauthable, :omniauth_providers => [:google, :facebook]

  def self.from_omniauth(auth)
    where(provider: auth.provider, uid: auth.uid).first_or_create do |user|
      user.email = auth.info.email
      user.password = Devise.friendly_token[0,20]
    end
  end

end

config.initializers.devise.rb

Devise.setup do |config|

  # Credentials
  config.omniauth :facebook, ENV['FACEBOOK_CLIENT_ID'], ENV['FACEBOOK_CLIENT_SECRET']
  config.omniauth :google_oauth2, ENV['GOOGLE_CLIENT_ID'], ENV['GOOGLE_CLIENT_SECRET'], name: 'google'

end

config.routes.rb

MyApplication::Application.routes.draw do
  devise_for :users, :controllers => { :omniauth_callbacks => "omniauth_callbacks" }
end

Gemfile

gem 'devise'

# ENV variable management
gem 'figaro'

# OmniAuth Authentication providers
gem 'omniauth-google-oauth2'
gem 'omniauth-facebook'

group :development, :test do
  gem 'capybara'
end

spec.controllers.omniauth_callbacks_controller_spec.rb

require 'rails_helper'

RSpec.describe OmniauthCallbacksController, :type => :controller do

  before(:each) do
    request.env['devise.mapping'] = Devise.mappings[:user]
  end

  describe 'Facebook' do

    context 'Success handling' do

      before(:each) do
        request.env['omniauth.auth'] = FactoryGirl.create(:auth_hash, :facebook)
        get :facebook
      end

      let(:user) { User.find_by(email: 'testuser@facebook.com') }

      it 'should set :notice flash' do
        expect(flash[:notice]).to exist
      end

      it 'should set current_user to proper user' do
        expect(subject.current_user).to eq(user)
      end

    end

    context 'Non-persisting User' do

      before(:each) do
        request.env['omniauth.auth'] = FactoryGirl.create(
            :auth_hash, :facebook, :does_not_persist
        )
        get :facebook
      end

      it 'should redirect to new user registration' do
        expect(response).to redirect_to new_user_registration_url
      end

      it 'should set flash :notice' do
        expect(flash[:notice]).to exist
      end

    end

  end

  describe 'Google' do

    context 'Success handling' do

      before(:each) do
        request.env['omniauth.auth'] = FactoryGirl.create(:auth_hash, :google)
        get :facebook
      end

      let(:user) { User.find_by(email: 'testuser@gmail.com') }

      it 'should set :notice flash' do
        expect(flash[:notice]).to exist
      end

      it 'should set current_user to proper user' do
        expect(subject.current_user).to eq(user)
      end

    end

  end

  describe 'Failure' do

    after(:each) do
      Rails.application.reload_routes!
    end

    before(:each) do

      Rails.application.routes.draw do
        devise_scope :user do
          get '/users/auth/failure' => 'omniauth_callbacks#failure'
        end
        root 'static_pages#index'
      end

      get :failure

    end

    it 'should redirectto root path' do
      expect(response).to redirect_to root_path
    end

    it 'should set flash :alert' do
      expect(flash[:alert]).to exist
    end

  end

end

spec.factories.omniauth.rb

FactoryGirl.define do
  # When calling auth_hash, use one of the traits listed below for a
  # facebook user, google user or when testing a user who does not
  # persist, use the does_not_persist trait.
  factory :auth_hash, class: OmniAuth::AuthHash do

    initialize_with do
      OmniAuth::AuthHash.new({
        provider: provider,
        uid: uid,
        info: {
          email: email
        }
      })
    end

    trait :facebook do
      provider "facebook"
      sequence(:uid)
      email "testuser@facebook.com"
    end

    trait :google do
      provider "google"
      sequence(:uid)
      email "testuser@gmail.com"
    end

    trait :does_not_persist do
      email ""
    end

  end
end

spec.rails_helper.rb

# This file is copied to spec/ when you run 'rails generate rspec:install'
ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment', __FILE__)
# Prevent database truncation if the environment is production
abort("The Rails environment is running in production mode!") if Rails.env.production?
require 'spec_helper'
require 'rspec/rails'
# Add additional requires below this line. Rails is not loaded until this point!

# Requires supporting ruby files with custom matchers and macros, etc, in
# spec/support/ and its subdirectories. Files matching `spec/**/*_spec.rb` are
# run as spec files by default. This means that files in spec/support that end
# in _spec.rb will both be required and run as specs, causing the specs to be
# run twice. It is recommended that you do not name files matching this glob to
# end with _spec.rb. You can configure this pattern with the --pattern
# option on the command line or in ~/.rspec, .rspec or `.rspec-local`.
#
# The following line is provided for convenience purposes. It has the downside
# of increasing the boot-up time by auto-requiring all files in the support
# directory. Alternatively, in the individual `*_spec.rb` files, manually
# require only the support files necessary.
#
# Dir[Rails.root.join('spec/support/**/*.rb')].each { |f| require f }

# Checks for pending migration and applies them before tests are run.
# If you are not using ActiveRecord, you can remove this line.
ActiveRecord::Migration.maintain_test_schema!

RSpec.configure do |config|
  # Remove this line if you're not using ActiveRecord or ActiveRecord fixtures
  config.fixture_path = "#{::Rails.root}/spec/fixtures"

  # If you're not using ActiveRecord, or you'd prefer not to run each of your
  # examples within a transaction, remove the following line or assign false
  # instead of true.
  config.use_transactional_fixtures = true

  # RSpec Rails can automatically mix in different behaviours to your tests
  # based on their file location, for example enabling you to call `get` and
  # `post` in specs under `spec/controllers`.
  #
  # You can disable this behaviour by removing the line below, and instead
  # explicitly tag your specs with their type, e.g.:
  #
  #     RSpec.describe UsersController, :type => :controller do
  #       # ...
  #     end
  #
  # The different available types are documented in the features, such as in
  # https://relishapp.com/rspec/rspec-rails/docs
  config.infer_spec_type_from_file_location!

  # Filter lines from Rails gems in backtraces.
  config.filter_rails_from_backtrace!

  # arbitrary gems may also be filtered via:
  # config.filter_gems_from_backtrace("gem name")

  # Simplify factory_girl syntax
  config.include FactoryGirl::Syntax::Methods

  # Clean up the test DB after each run by truncating all the tables
  config.after :suite do
    DatabaseCleaner.clean_with :truncation
  end

  # Turn on "test mode" for OmniAuth
  OmniAuth.config.test_mode = true

  # Include Devise TestHelpers
  config.include Devise::TestHelpers, type: :controller
end