是否可以在 Firestore 中比较两个 collections？

Question

I'm developing an Android app with Java and using Firestore, It's a social.network and I have a collection with all the posts. I'm trying to show only those posts that belong to the followed users, so I make a query to show all the posts ordered by timestamp, but I don't know if I can filtered them by comparing with the collection "followed" inside "User".

The main collection "Users" has documents, each of them is a user, inside every user there is a subcollection "followed" that contains the followed users, every document is a user and the document id is the same that the User ID.

The posts are stored in another main collection called Posts, so I need to compare the id User inside "Posts" documents with the id of the docs in the subcollection "followed". I hope somebody can help me, I spent a lot of time and I can't find anything, thank you.

Answer 1

Firestore does not have the ability to "join" documents in collections as you're describing here. It's relatively straightforward in SQL (if your server has enough memory), but Firestore (and other NoSQL databases) aren't built for this, due to its distributed nature, and the way it needs to scale.

The only way to do what you want is to write code to read every document in every collection that would need a comparison, and also perform that comparison with the documents in memory.