title: Document tagging in MongoDB
body: <p>If I have millions (or even billions) of objects in this scheme:</p>
  
  <pre><code>{ id: ...,
  name: ...,
  desc: ... }
  </code></pre>
  
  <p>And I want to add tags to each of those objects, what would be the most efficient way to do this?</p>
  
  <p>Later, I want to be able to count objects for each tag, see all objects that have tag X, etc.</p>