Rails 4 OmniAuth with Devise for Twitter and Facebook

gem 'devise'
gem 'omniauth'
gem 'omniauth-twitter'
gem 'omniauth-facebook'

rails generate devise:install
rails generate devise user
rails g model identity provider:string uid:string user:references

class Identity < ActiveRecord::Base
  belongs_to :user

  validates :uid, :provider, presence: true
  validates :uid, uniqueness: { scope: :provider }

  def self.find_for_oauth(auth)
    find_or_create_by(uid: auth.uid, provider: auth.provider)
  end
end

Devise.setup do |config|
  ...
  config.omniauth :facebook, "KEY", "SECRET"
  config.omniauth :twitter, "KEY", "SECRET"
end

devise_for :users, controllers: { omniauth_callbacks: 'omniauth_callbacks' }

class OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def provider
    auth = request.env["omniauth.auth"]
    @user = User.find_for_oauth(auth, current_user)

    if @user.persisted?
      sign_in_and_redirect @user, event: :authentication
      set_flash_message(:notice, :success, kind: "#{provider}".capitalize) if is_navigational_format?
    else
      session["devise.#{provider}_data"] = env["omniauth.auth"]
      redirect_to new_user_registration_url
    end
  end

  alias_method :twitter, :provider
  alias_method :facebook, :provider

  def after_sign_in_path_for(resource)
    if resource.email_verified?
      super resource
    else
      finish_signup_path(resource)
    end
  end
end

class User < ActiveRecord::Base
  TEMP_EMAIL_PREFIX = 'change@me'
  TEMP_EMAIL_REGEX = /\Achange@me/

  devise :database_authenticatable, :registerable, :confirmable,
    :recoverable, :rememberable, :trackable, :validatable, :omniauthable

  validates_format_of :email, :without => TEMP_EMAIL_REGEX, on: :update

  def self.find_for_oauth(auth, signed_in_resource = nil)
    identity = Identity.find_for_oauth(auth)

    user = signed_in_resource ? signed_in_resource : identity.user

    if user.nil?
      # Get the existing user by email if the provider gives us a verified email.
      # If no verified email was provided we assign a temporary email and ask the
      # user to verify it on the next step via UsersController.finish_signup
      email_is_verified = auth.info.email && (auth.info.verified || auth.info.verified_email)
      email = auth.info.email if email_is_verified
      user = User.where(:email => email).first if email

      # Create the user if it's a new registration
      if user.nil?
        user = User.new(
          name: auth.extra.raw_info.name,
          email: email ? email : "#{TEMP_EMAIL_PREFIX}-#{auth.uid}-#{auth.provider}.com",
          password: Devise.friendly_token[0,20]
        )

        # check if confirmable is enabled
        user.skip_confirmation! if user.respond_to?(:skip_confirmation)
        user.save!
      end
    end

    if identity.user != user
      identity.user = user
      identity.save!
    end

    user
  end

  def email_verified?
    self.email && self.email !~ TEMP_EMAIL_REGEX
  end
end