http://blog.guiltry.com/devise-multiple-provider-with-factory-method-pattern

omniauth_callbacks_controller.rb

class OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def self.provides_callback_for(provider)
    
    class_eval %Q{
      def #{provider}
        # NOTE:
        # Extract Oauth logic to a new ruby class.
        @user = FactoryMethod::User.create("#{provider}", env["omniauth.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
    }
  end

  [:facebook, :google_oauth2].each do |provider|
    provides_callback_for provider
  end
end