Create a transaction to update multiple places in Firebase Database

Question

I'm making an app for an (imaginary) massage center. In an activity, the user can click a button to make a reservation. Only one customer must be able to make a reservation for a specific date and time, so I have to use a transaction that takes into account the current state of the database to handle the update.

Right now, I am doing the following:

public void processViewResult(Object object, String tag) {

if (tag.equals("reservation")) {

    final Reservation reservation = (Reservation) object;

    database.getReference().runTransaction(new Transaction.Handler() {
        @Override
        public Transaction.Result doTransaction(MutableData mutableData) {
            MutableData freeSlots = mutableData.child("slots").child(reservation.getDate());
            MutableData userReservations = mutableData.child("users")
                    .child(reservation.getUserId()).child("reservations");
            Boolean isSlotAvailable = freeSlots.hasChild(reservation.getHour());

            if (isSlotAvailable) {
                mutableData.child(reservation.getHour()).setValue(null);
                userReservations.child(String.valueOf(reservation.hashCode())).setValue(reservation);
                return Transaction.success(mutableData);
            } else {
                return Transaction.abort();
            }
        }

        @Override
        public void onComplete(DatabaseError databaseError, boolean b, DataSnapshot dataSnapshot) {
            if (databaseError != null) {
                Log.e(this.getClass().getSimpleName(), databaseError.toString());
            } else {
                Log.e(this.getClass().getSimpleName(), "Updated successfully");
            }
        }
    });

}

}

There is several problems with this approach:

1) Since I need to update both the time slots available and the reservations made for the current customer, I need to get a snapshot of the whole database. If the application scaled, this would be very inefficient.

2) This approach has a lot of boilerplate.

Can you please tell me if there is a simpler and more elegant solution to this?