title: Create a doc. if it does not exist in Mongo
body: <p>I'm using Mongo 3.2.10 now. I would like to create a document on demand like that:</p>
  
  <ol>
  <li>retrieve a document from a collection by id</li>
  <li>create the doc. if it does not exist</li>
  </ol>
  
  <p>Can I do (1) and (2) <em>atomically</em> with Mongo 3.2.10 ? </p>