title: Updating a sub-document in mongodb?
body: <p>How do I target an subdocument in the <code>authors</code> array as shown below, in order to update it?</p>
  
  <pre><code>collection.update({'_id': "4f44af6a024342300e000001"}, {$set: { 'authors.?' }} )
  </code></pre>
  
  <p>The document:</p>
  
  <pre><code>{
      _id:     "4f44af6a024342300e000001",
      title:   "A book", 
      created: "2012-02-22T14:12:51.305Z"
      authors: [{"_id":"4f44af6a024342300e000002"}] 
  }
  </code></pre>