List online users with devise in rails

I just add a last_sign_out_at column to my Users table and then subclassed the Devise sessions controller so I could override the destroy method to set it when the session is destroyed (user signs out):

# app/controllers

class SessionsController < Devise::SessionsController

  def destroy
    current_user.update_attribute(:last_sign_out_at, Time.now)
    super
  end

end 

And then in my User model I have a method to check if the user is online:

class User < ActiveRecord::Base

  def online?
    if current_sign_in_at.present? 
      last_sign_out_at.present? ? current_sign_in_at > last_sign_out_at : true
    else
      false
    end
  end

end

Also you need to tell Devise to use the new Sessions controller in your routes.