Hierarchic structure Firestore and subcollections

Question

I am trying to build a database with Cloud Firestore and having read the documentation about hierarchic structure I have found another solution to the one present in the documentation example.

I have a collection of categories, and each category can have subcategories. Whereas I could use the system provided in the Firestore documentation example such as collection/document/subcollection/document... I have found another example for MongoDB where instead of having subcollections it uses nested data in a single document with parent-child relations as described in the image below

What approach is better if I want the user to be able to see all the subcategories so the user can categorize a certain event? By better I mean in terms of avoiding multiple connections to the DB (as this is what Firestore prices with).

Answer 1

This document discusses nested data and its tradeoffs, "Choose a Data Structure". I'm not sure that the statement about multiple connections for pricing is correct. Firestore charges for number of operations, storage, and network bandwidth.

If all users share a single categories list, you would be able to retrieve the entire list with one operation with your current structure. Note that there is a 1 MiB limit for document size.