title: How to get pointer of current document to update in updateMany
body: <p>I have a latest mongodb 3.2 and there is a collection of many items that have timeStamp.</p>
  
  <p>A need to convert milliseconds to Date object and now I use this function:</p>
  
  <pre><code>db.myColl.find().forEach(function (doc) {
      doc.date = new Date(doc.date);
      db.myColl.save(doc);
  })
  </code></pre>
  
  <p>It took very long time to update 2 millions of rows.</p>
  
  <p>I try to use updateMany (seems it is very fast) but how I can get access to a current document? Is there any chance to rewrite the query above by using updateMany?</p>
  
  <p>Thank you.</p>