title: friendly_id thowing error while creating a new user
body: <p>I have a model User which is polymorphically associated with Person and Company model.</p>
  
  <p>I am using friendly_id to show permalinks for both Person and Company.</p>
  
  <p>Person has first_name attribute and I put the following in my model.</p>
  
  <pre><code>extend FriendlyId
  friendly_id :first_name
  </code></pre>
  
  <p>Now when I update my existing entries then the slug gets created perfectly. But issue arises when I try creating a new use which throws a routing error.</p>
  
  <p>Secondly, every person has one user and user has a username, so when I try this following command, then the update and new record both throws unknown method username for nil class error.</p>
  
  <pre><code>extend FriendlyId
  friendly_id :person_permalink, use: [:slugged, :history]
  
  def person_permalink
    "#{self.user.username} #{self.name}"
  end
  </code></pre>