Cloud Firestore: How to set a field in document to null

Question

I use a collection of documents that I use to identify user groups. My intention is to only fill in the document id field with the user id without having other useless fields in

Answer 1

Assuming that the the userId property is of type String, please use the following code in order to update all users with the userId = null:

FirebaseFirestore rootRef = FirebaseFirestore.getInstance();
CollectionReference usersRef = rootRef.collection("users");
usersRef.get().addOnCompleteListener(new OnCompleteListener() {
    @Override
    public void onComplete(@NonNull Task task) {
        if (task.isSuccessful()) {
            List list = new ArrayList<>();
            for (DocumentSnapshot document : task.getResult()) {
                list.add(document.getId());
            }

            for (String id : list) {
                rootRef.collection("Users").document(id).update("userId", null).addOnSuccessListener(new OnSuccessListener() {
                    @Override
                    public void onSuccess(Void aVoid) {
                        Log.d(TAG, "Username updated!");
                    }
                });
            }
        }
    }
});

If you are using a model class for your user, please see my answer from this post.