从 Firestore 中的多个子集合的多个子集合中获取所有文档的最有效方法是什么？

Question

Let's say I have a collection with a structure as such:

Collection	Documents	Sub-Collection	Documents
users	user1	invoices	inv1
			inv2
			inv3
	user2	invoices	inv1
			inv2
			inv3
	user3	invoices	inv1
			inv2
			inv3

Let's say I have over 200 users with each user having over 500 invoices.

I am using Firestore as a backend for my ReactJS app. So I currently have the ability to pull each user's invoices when I access their userId . However, I want to have a new feature on a new page where I can access ALL INVOICES at once. So what is the most efficient way for me to do this?

A couple of scenarios that I could think of:

1. Create a cloud function where it compiles all Invoices from each user into a single new user with the name All Users .
2. Iterate through all the userId to get all of the invoices from each user.

Answer 1

I want to have a new feature on a new page where I can access ALL INVOICES at once. So what is the most efficient way for me to do this?

If you want to get all invoices of all users, then you should consider using a collection group query .

Since you say that:

Let's say I have over 200 users with each user having over 500 invoices.

Then we speak about 100k documents, which is way too much to get in a single go. For that, I recommend you get the data in smaller chunks by implementing pagination .

Regarding the first scenario, it will work, but it doesn't make any sense to do that since we have collection group queries.

Regarding the second scenario, it will also work but you need to store somehow all user IDs and then perform a separate query for each user. It complicates things a bit.