Create a collection inside users document: Cloud Firestore

Question

I'm trying to create an "Orders" collection inside the user document so that I can be able to get the orders that a specific user has made. This is what I tried:

if(firebaseUser!=null){
    String userID= firebaseUser.getUid();
    DocumentReference userDoc= db.collection("users").document(userID);
    userDoc.collection("orders").document(model.getMeal_name() + " " + day)
            .set(order).addOnSuccessListener(new OnSuccessListener<Void>() {
                @Override
                public void onSuccess(Void unused) {
                    Log.d(TAG, "Order entry into database");
                    if (progressDialog.isShowing()) {
                        progressDialog.dismiss();
                    }
                }
            });
}

But it's creating the document in the users collection as opposed to creating it inside a document. What am I doing wrong?

Here's my database screenshot with the new document it created.