omniauth_callbacks_controller.rb

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
  Devise.omniauth_configs.keys.each do |provider|
    define_method(provider.to_s) { omniauth_callback(provider) }
  end

  def failure
    redirect_to root_path
  end

  private

  def omniauth_callback(provider)
    auth_data = request.env["omniauth.auth"]

    user = SigninWithOmniauth.new(auth_data).execute

    if user.present? && user.persisted?
      sign_in_and_redirect user, event: :authentication
      set_flash_message(:notice, :success, kind: provider.to_s.capitalize) if is_navigational_format?
    else
      session["devise.oauth_data"] = auth_data.except(:extra)
      redirect_to new_user_session_path
    end
  end
end

signin_with_omniauth.rb

class SigninWithOmniauth

	def initialize(auth_data)
	  @auth_data = auth_data
	end

	def execute
    authentication = Authentication.from_omniauth(auth_data)

    raise(AuthError, I18n.t(:failed, scope: [:messages, :auth])) unless authentication.persisted?

    if authentication.user.nil?
    	attach_authentication_to_user(authentication, existing_user || new_user)
    end

		authentication.user
	end

	private

	def existing_user
		User.find_by_email(auth_data.info.email)		
	end

	def new_user
    User.create(name: auth_data.info.name, email: auth_data.info.email) do |user|
    	user.password = Devise.friendly_token[0,20]
    	user.skip_confirmation!
    end
	end

	def attach_authentication_to_user(authentication, user)
		authentication.update_column(:user_id, user)
	end

	attr_reader :auth_data
end