Gist for article: https://medium.com/evenbit/on-collision-course-with-cloud-firestore-7af26242bc2d

aggregate_reviews.ts

export const countReviews = functions.firestore.document("movies/{movieId}/reviews/{reviewId}").onCreate(async (event) => {
    console.log(`Got a ${event.data.data().stars} star review`);

    const review = event.data.data();
    const movieRef = admin.firestore().collection("movies").doc(event.params.movieId);
    const movie = (await movieRef.get()).data();

    const newNumReviews = movie.numReviews + 1;
    const newTotalScore = movie.totalReviewScore + review.stars;
    
    console.log(`[${event.data.id}] Got a ${review.stars} star review from ${review.name} (now ${movie.numReviews} total reviews)`);

    return movieRef.update({
        numReviews: newNumReviews,
        totalReviewScore: newTotalScore,
        averageScore: (newTotalScore/newNumReviews).toPrecision(3)
    });
});

create_reviews.ts

const numReviews = 20;
const promises = [];
for (let i = 0; i < numReviews; i++) {
    const numStars = Math.trunc(Math.random() * 5) + 1;
    console.log(`#${i} creating a ${numStars} star review.`);
    promises.push(admin.firestore().collection("movies").doc("tt0081748").collection("reviews").add({
        name: `Jane Deer #${i}`,
        stars: numStars,
        comment: "Meh"
    }));
}

await Promise.all(promises);