title: What should be used as User id?
body: <p>Flask-login required me to provide the following callback:</p>
  
  <pre><code>@login_manager.user_loader
  def load_user(user_id):
      return User.get(user_id)
  </code></pre>
  
  <p>But what should be used as id? Can I just use username? If yes, what should I return in case of user_loader("admin").is_authenticated()? How do I know if he was authenticated in the previous session? Should I keep track of it manually?</p>