FirebaseFirestore rootRef = FirebaseFirestore.getInstance();
    CollectionReference bulletinRef = rootRef.collection("facultades").document().collection("escuelas");
    bulletinRef.get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            if (task.isSuccessful()) {
                for (QueryDocumentSnapshot document : task.getResult()) {
                    Log.d(TAG, document.getString("nombre"));
                }
            } else {
                Log.d(TAG, "Error getting documents: ", task.getException());
            }
        }
    });