Handling conflict in find, modify, save flow in MongoDB with Mongoose

I would like to update a document that involves reading other collection and complex modifications, so the update operators in findAndModify() cannot serve my purpose.

Here's what I have:

Collection.findById(id, function (err, doc) {

    // read from other collection, validation
    // modify fields in doc according to user input
    // (with decent amount of logic)

    doc.save(function (err, doc) {
      if (err) {
        return res.json(500, { message: err });
      }

      return res.json(200, doc);
    });
}

My worry is that this flow might cause conflict if multiple clients happens to modify the same document.
It is said here that:

Operations on a single document are always atomic with MongoDB databases

I'm a bit confused about what Operations mean.

• Does this means that the findById() will acquire the lock until doc is out of scope (after the response is sent), so there wouldn't be conflicts? (I don't think so)
• If not, how to modify my code to support multiple clients knowing that they will modify Collection?
• Will Mongoose report conflict if it occurs?
• How to handle the possible conflict? Is it possible to manually lock the Collection?
  • I see suggestion to use Mongoose's versionKey (or timestamp) and retry for stale document
  • Don't use MongoDB altogether...

Thanks.

EDIT

Thanks @jibsales for the pointer, I now use Mongoose's versionKey (timestamp will also work) to avoid committing conflicts.

aaronheckmann — Mongoose v3 part 1 :: Versioning

See this sample code:
https://gist.github.com/anonymous/9dc837b1ef2831c97fe8