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

countReviews.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);
    
    admin.firestore().runTransaction(async transaction => {
        const movie = (await transaction.get(movieRef)).data();

        movie.numReviews += 1;
        movie.totalReviewScore += review.stars;
        movie.averageScore = (movie.totalReviewScore/movie.numReviews).toPrecision(3);
        
        console.log(`[${event.data.id}] Got a ${review.stars} star review from ${review.name} (now ${movie.numReviews} total reviews)`);

        return transaction.update(movieRef, movie);
    });
});