title: Duplicate a document in MongoDB using a new _id
body: <p>Ok, I suppose that this is a silly question and probably has a simple answer.</p>
  
  <p>How can I duplicate a document in MongoDB, changing the _id of the new one?</p>
  
  <p>Imaging that you have the original document:</p>
  
  <pre><code>&gt; var orig = db.MyCollection.findOne({_id: 'hi'})
  </code></pre>
  
  <p>And now I want another document in the collection with _id 'bye'.  </p>