Free Code Camp's new deserializer

deserializer.js

const fields = {
  progressTimestamps: false,
  completedChallenges: false,
  challengeMap: false
};
passport.deserializeUser((id, done) => {
  // We use a combineLatest here to run both of these
  // database request in parallel
  Observable.combineLatest(
    // `findById$` notice the dolar sign?
    // It's a convention we use to indicate that
    // this function returns an Observable
    // DB query 1:
    // This request attempts to find the user
    // You may notice the second arg, it is an object
    // with the `fields` property populated by the object
    // defined above. This tells loopback we want to filter out those
    // fields from the request
    this.userModel.findById$(id, { fields }),
    // DB query 2: Some secret souce :)
    this.userModel.getPointsById$(id),
    // Attach those points to the user object when
    // both queries have returned
    (user, points) => {
      if (user) { user.points = points; }
      return user;
    }
  )
    .doOnNext(user => {
      // make sure the user actually exists
      // if not, then we throw an error. This is caught by RxJS
      // and sent down the error pipeline
      if (!user) { throw new Error('deserialize found no user'); }
    })
    .subscribe(
      user => done(null, user),
      // this is where errors are sent
      // to be handled downstream by our custom
      // express error handlers
      done
    );
});