title: add to nested json in mongoDB
body: <p>For an instance I have a json document in mongodb </p>
  
  <pre><code>{
  a:{b:c}
  }
  </code></pre>
  
  <p>Now what am I suppost to do if I want to add to a nested json which will make it look like:</p>
  
  <pre><code>{
  a:{b:c, d:e}
  }
  </code></pre>
  
  <p>I have read the documentation of <code>update</code>. It has documented about <code>$inc</code>, <code>$rename</code>, <code>$set</code> &amp; <code>$unset</code>. But I want it to add with existing elements of <code>a</code>. just like <code>$addToSet</code> do with an array.</p>