devise/test/rails_app/app/controllers/users/omniauth_callbacks_controller.rb

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def facebook
    data = env["omniauth.auth"]
    session["devise.facebook_data"] = data["extra"]["user_hash"]
    render :json => data
  end

  def sign_in_facebook
    user = User.find_by_email('user@test.com')
    user.remember_me = true
    sign_in user
    render :text => ""
  end
end