title: How does $unset work?
body: <p>In the <a href="https://docs.mongodb.com/manual/reference/method/db.collection.update/#remove-fields" rel="nofollow">mongo documentation</a> unsetting a field is done with <code>$unset</code>. I can't quite grasp how it works, but it seems like it should be simple.</p>
  
  <pre><code>The following operation uses the $unset operator to remove the tags field:
  
  db.books.update( { _id: 1 }, { $unset: { tags: 1 } } )
  </code></pre>
  
  <p>My confusion arises when setting what to unset. What is the value <code>1</code> for in the <code>$unset</code> clause?</p>