title: has_one still returning nil after updating belongs_to counterpart
body: <p>I have a user model with <code>has_one :designpad</code>, a designpad model with <code>belongs_to :user</code> and the column <code>user_id</code>.</p>
  
  <p>Now I do this:</p>
  
  <pre><code>u = User.first
  d = Designpad.new
  d.user_id = u.id
  d.save
  
  d.user # returns the user (== u)
  u.designpad # always returns nil, but why?
  </code></pre>
  
  <p>Why does <code>u.designpad</code> return nil? <code>u.methods</code> lists the methods <code>designpad</code> and <code>designpad=</code>, just as expected while using a <code>has_one</code> association.</p>
  
  <p>But <code>u.designpad</code> just won't return the expected designpad.</p>