app/controllers/users/omniauth_callbacks_controller.rb
class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
def facebook
@user = User.find_or_create_for_facebook_oauth(request.env["omniauth.auth"])
if @user.persisted?
sign_in_and_redirect @user, :event => :authentication
set_flash_message(:notice, :success, :kind => "Facebook") if is_navigational_format?
else
session["devise.facebook_data"] = request.env["omniauth.auth"]
redirect_to new_user_registration_url
end
end
end