title: Mongoose retrieving data without _id field
body: <p>I would like to retrieve some data from a Mongoose setting in my Node.js application. I noticed that no matter what I write as field selection, I always get the <code>_id</code> field. Is there a way not to fetch it?
  This is how I do right now:</p>
  
  <pre><code>Transaction.find({username : user.username}, ['uniqueId', 'timeout', 'confirmation_link', 'item_name'], function(err, txs){
          console.log("user : " + user.username + " with txs: " + txs);
          callback(txs);
  });
  </code></pre>
  
  <p>And logs me the results which contain the <code>_id</code> field.</p>