tracker-saving.js

// pseudo code for creating a new tracker and saving to the user that created it
(req, res) => {

  const userId = req.body.userId;
  const trackerData = {
    name: req.body.name,
    description: req.body.name,
    // more stuff
  }
  let trackerId;

  // I don't even think that's the right save method
  Tracker.save(trackerData).then((trackerDoc) => { // save the new tracker and get the saved document from mongo
    trackerId = trackerDoc._id;  // get the id that was just created by the database
    return User.findById(userId);  // return the promise from finding the user by id
  }).then((userDoc) => { // get the user doc from the find
    userDoc.trackerIds.push(trackerId);  // add the new tracker id to the user that created it
    userDoc.save();  // save it (this probably isn't the right syntax)
  })
}