title: Laravel - How to check if One To One relationship exists?
body: <p>Let's say I have 2 models: Car, Driver.</p>
  
  <p>Car model:</p>
  
  <pre><code>&lt;?php
  
  class Car extends \Eloquent {
      public function driver()
      {
          return $this-&gt;belongsTo('Driver');
      }
  }
  </code></pre>
  
  <p>Driver model:</p>
  
  <pre><code>&lt;?php
  
  class Driver extends \Eloquent {
      public function car()
      {
          return $this-&gt;hasOne('Car');
      }
  }
  </code></pre>
  
  <p>How to query one or all cars that don't belong to any driver yet?
  How to check whether queried car belongs to any driver?</p>