Retrieve data from Firestore in a way that is faster and more cost effective with data aggregation. In this episode, we use Firebase Cloud Functions to read data from a sub-collection and write it to its parent document. https://angularfirebase.com/lessons/firestore-cloud-functions-data-aggregation/
Firestore: https://firebase.google.com/docs/firestore/
NoSQL Aggregation: https://www.thoughtworks.com/insights/blog/nosql-databases-overview