title: Mongoose query where value is not null
body: <p>Looking to do the following query:</p>
  
  <pre><code>Entrant
      .find
        enterDate : oneMonthAgo
        confirmed : true
      .where('pincode.length &gt; 0')
      .exec (err,entrants)-&gt;
  </code></pre>
  
  <p>Am I doing the where clause properly? I want to select documents where <code>pincode</code> is not null.</p>