Mongoose: how do I update/save a document?

I need to save a document to a mongo collection.
I want to save the 'insertedAt' and 'updatedAt' Date fields, so I suppose I can't do it in one step...

This is my last try:

  my topic = new Topic(); // Topic is the model
  topic.id = '123'; // my univocal id, !== _id
  topic.author = 'Marco';
  ...

  Topic.findOne({ id: topic.id }, function(err, doc) {
    if (err) {
      console.error('topic', topic.id, 'could not be searched:', err);
      return false;
    }
    var now = new Date();
    if (doc) { // old document
      topic.updatedAt = now;
    } else { // new document
      topic.insertedAt = now;
    }
    topic.save(function(err) {
      if (err) {
        console.error('topic', topic.id, 'could not be saved:', err);
        return false;
      }
      console.log('topic', topic.id, 'saved successfully');
      return true;
    });
  });

But this way I end up duplicating records... :-(

Any suggestion?

via MarcoS