title: how to $project ObjectId to string value in mongodb aggregate?
body: <p>Is there an operator I could use in <strong>aggregate</strong> function to get a string instead of ObjectId in response? </p>
  
  <pre><code>db.something.aggregate([
    { "$match": { "property": { "$exists": true } } },
    { "$project": { "stringId": "$_id.???" } }
  ])
  </code></pre>