title: Move "property: value" position in MongoDB
body: <p>I've been wondering if you could move one field position. For example:</p>
  <p><strong>Current Document:</strong></p>
  <pre class="lang-js prettyprint-override"><code>{
      user_id: 1234567890,
      coins: 100,
      card: 500,
      username: 'Shin'
  }
  </code></pre>
  <p><strong>(Expected Document)</strong> I would like to move the username into this:</p>
  <pre class="lang-js prettyprint-override"><code>{
      user_id: 1234567890,
      username: 'Shin',
      coins: 100,
      card: 500
  }
  </code></pre>