Firebase Firestore query by subcolletion length

Question

I am trying to query my Firestore collection (in Node.js for my flutter app), and to get the 10 documents which has the most objects in their subcolllection called Purchases (to get the best sellers).

Is it possible in Firestore? Or do I have to keep an int field outside of my subcollection to represent its length?

Thank you!

Answer 1

I thought this was answered recently, but can't find it right now, so...

Firestore queries (and other read operations) work on a single collection, or a group of collections with the same name. They don't consider any data in other (nested or otherwise) collections, nor can they query based on aggregates (such as the number of documents), unless those aggregates are stored in a document in the queried collection.

So the solution is indeed to keep a counter in a document in the collection you are querying against, and updating that counter with every add/delete to the subcollection.