mongoose atomicity question

gistfile1.js

// add comment function
function addComment(req, res) {

  PostModel.findById(req.params.postId, function (err, foundPost) {
    if(err) {
      return handleError(err)
    }

    var commentObj = {
      user: req.session.userId,
      comment : req.body.commentText
    }

    foundPost.comments.push(commentObj)

    foundPost.save(function (err) {
      if (err) {
        return handleError(err)
      }
      else{
        // want to get the comment I just added
        res.send(foundPost.comments[foundPost.comments.length-1])
      }
    })
  
  })

}

// schemas
var Comment = new Schema({
  user: {type: Schema.ObjectId, required: true},
  comment : {type: String, required: true},
  timestamp: {type: Date, required: true, default: Date.now}
})

var Post = new Schema({
  timestamp: {type: Date, default: Date.now, required: true},
  user: {type: Schema.ObjectId, required: true},   //id of the responder
  text: {type: String},
  comments: {type: [Comment]},
})