My idea is to create a like&unlike system for posts (a user can like and unlike a post) and to show a list of users in a fragment, ordered this way: the top user is the one for which there are the most numerous likes.
In my Firebase Firestore database:
I have a collection of posts. Each document (which is a post) contains the ID of the user that created it, and a counter of likes (I call it "A").
I have another collection, of users. Each document (which is a user) contains a counter (I call it "B") that counts for the number of likes, all posts combined.
For the moment, I use "B" to order the users in the list of users I've described above. I use "B" this way: return FirebaseFirestore.getInstance().collection("users").orderBy("likes", Query.Direction.DESCENDING);.
However, due to some technical facts in relation to Firebase Security Rules, and to simplify the database structure, I would want to only use "A", and remove "B". More precisely, I would want to get all the users, and order them according to "A". However, the counter "A" is defined in the documents of the collection of posts, whereas the counter "B" was defined in the documents of the collection of users: that makes it easier to use.
So my question is: for each user, I would have to get all their posts, then sum up their counter "A", then get all the users, then order them according to this sum. However, I would want to know: is it possible to do this using Firebase Firestore Android API? (something like the instruction given before) In others words: How to order documents from a collection, according to the field of documents of another collection? But note that in fact it's more difficult, since I must sum up the counter "A" of each post created by the user.
