Mongoose .populate() after save using promises (node.js)

Question

Yes, I have read Mongoose populate after save, but I still can't seem to put the pieces together with promises.

I have tried every different way of using .populate in pushComment() that I can think of, and cannot seem to get it right. Any help is greatly appreciated, Thank you.

endpoint

export function addComment(req, res) {
  let newComment = {
    author: req.user._id,
    body: req.body.comment,
    date: new Date()
  };
  return Deal.findById(req.params.id)
    .exec()
    .then(handleEntityNotFound(res))
    .then(pushComment(newComment))
    .then(respondWithResult(res))
    .catch(handleError(res));
}

pushComment

function pushComment(comment) {
  return function (entity) {
    entity.comments.push(comment);
      return entity.populate('author').save()
      .then(updated => {
        return updated
      });
  };
}

respondWithResult

function respondWithResult(res, statusCode) {
  statusCode = statusCode || 200;
  return function (entity) {
    if (entity) {
      res.status(statusCode).json(entity);
    }
  };
}