public static List<User> getPayers(String bid) {
        List<User> users = new ArrayList<>();

        //asynchronously retrieve all documents
        ApiFuture<QuerySnapshot> future = db.collection("debts").document(bid).collection("payees").get();
        // future.get() blocks on response
        List<QueryDocumentSnapshot> documents = null;
        try {
            documents = future.get().getDocuments();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
        for (QueryDocumentSnapshot document : documents) {
            System.out.println(document.getId() + "\n");
            User u = new User(document.getId(), document.get("name").toString());
            users.add(u);
        }

        return users;
    }

    public static int getNumPayers(String bid) {
        return getPayers(bid).size();
    }

    public static List<String> getAllBids(String myUid) {
        List<String> bids = new ArrayList<>();

        //asynchronously retrieve all documents
        ApiFuture<QuerySnapshot> future = db.collection("debts").get();
        // future.get() blocks on response
        List<QueryDocumentSnapshot> documents = null;
        try {
            documents = future.get().getDocuments();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

        for (QueryDocumentSnapshot document : documents) {
            if (document.get("uid").toString().equals(myUid)) {
                System.out.println(document.getId() + "\n");
                bids.add(document.getId());
            }
        }

        return bids;
    }

    public static List<User> getAllPayers(String myUid) {
        List<String> bids = getAllBids(myUid);
        List<User> users = new ArrayList<>();
        for (String s: bids) {
            users.addAll(getPayers(s));
        }

        return users;
    }