How to get all subcollections from Firestore documents?

Question

I have collection users with documents and subcollections records in each of them. How I can get list of all subcollections of records and then merge it in one global collection? How this can be done?

I've listener to get documents from one subcollection:

func recordsObserve(records: [MRecords], completion: @escaping (Result<[MRecords], Error>) -> Void) -> ListenerRegistration? {
    var records = records
    let recordsRef = db.collection(["users", currentUserId, "records"].joined(separator: "/"))
    let recordsListener = recordsRef.addSnapshotListener { (querySnapshot, error) in
        guard let snapshot = querySnapshot else {
            print(1)
            completion(.failure(error!))
            return
        }
        
        snapshot.documentChanges.forEach { (diff) in
            guard let record = MRecords(document: diff.document) else { return }
            switch diff.type {
            case .added:
                guard !records.contains(record) else { return }
                records.append(record)
            case .modified:
                guard let index = records.firstIndex(of: record) else { return }
                records[index] = record
            case .removed:
                guard let index = records.firstIndex(of: record) else { return }
                records.remove(at: index)
            }
        }
        
        completion(.success(records))
    }
    
    return recordsListener
}

But I need listener to get all subcollections of records from all users.

Answer 1

What you're looking for is known as a collection group query in Firestore. To get all documents from all records collection, it'd look something like this:

db.collectionGroup("records").getDocuments { (snapshot, error) in
    // ...
}

While looping over the documents in the snapshot, you can get the document ID of the user document by using the parent property of the record's DocumentReference and CollectionReference .