title: Mongo: updating document and adding/updating lastModified
body: <p>I need to update a document matching item.id</p>
  
  <p>Fields to update the document are stored in the object item</p>
  
  <p>Query:</p>
  
  <pre><code>bulk.find({ id: item.id }).upsert().updateOne({ $set: item, $currentDate : {lastModified: true} })
  
  </code></pre>
  
  <p>{$set: item} - does the job and adds all fields from the item to the document, however when I add $currentDate field, mongo returns: </p>
  
  <pre><code> errmsg: "Updating the path 'lastModified' would create a conflict at 'lastModified'",
  
  </code></pre>
  
  <p>Should I just do item.lastModified = (new Date()).getTime(), or I can do it via query?</p>