title: The $sort key specification must be an object
body: <p>I am trying to sort documents in my collection by a field that is a date object:</p>
  
  <pre><code>MyModel.aggregate(
      [
          { "$skip": ( page.current-1 ) * 15 },
          { "$limit": 15 },
          { "$sort": "$personal.dateObject" }
      ], function..................... etc
  </code></pre>
  
  <p>And i am receiving this error:</p>
  
  <pre><code>{ [MongoError: exception:  the $sort key specification must be an object]
    name: 'MongoError',
    errmsg: 'exception:  the $sort key specification must be an object',
    code: 15973,
    ok: 0 }
  </code></pre>
  
  <p>I don't know why, cause the personal.dateObject, is a object:</p>
  
  <p>Example:</p>
  
  <pre><code>   "personal": {
          "dateObject": {
              "$date": "2015-11-20T02:00:00.000Z"
          },
  </code></pre>
  
  <p>Any ideas??</p>