title: Populating on an already fetched document. Is it possible and if so, how?
body: <p>I have a document fetched as:</p>
  
  <pre><code>Document
    .find(&lt;condition&gt;)
    .exec()
    .then(function (fetchedDocument) {
      console.log(fetchedDocument);
    });
  </code></pre>
  
  <p>Now this document has a reference to another document. But I am not populating that reference when I'm querying this document. Instead, I wanna populate it later. So is there any way of doing that? Can I do this:</p>
  
  <pre><code>fetchedDocument
    .populate('field')
    .exec()
    .then(function (reFetchedDocument) {
      console.log(reFetchedDocument);
    });
  </code></pre>
  
  <p>Another method I came across is to do this:</p>
  
  <pre><code>Document
    .find(fetchedDocument)
    .populate('field')
    .then(function (reFetchedDocument) {
      console.log(reFetchedDocument);
    });
  </code></pre>
  
  <p>Now does this re-fetch the entire document all over again or just fetch the populated part and add it in?</p>