title: Check if association exists
body: <p>I've got a User model like this :</p>
  
  <pre><code>has_one :car
  </code></pre>
  
  <p>and Car model :</p>
  
  <pre><code>belongs_to :user
  </code></pre>
  
  <p>How can I check which users don't have cars?</p>