public Completable addContribution(Piggy piggy, Contribution contribution) {
        Completable updatePiggy = Completable.fromCallable(() -> {
            piggy.includeContribution(contribution.getAmount());
            return piggyRepository.updateWithCallback(piggy);
        });

        Completable insertContribution = Completable.fromCallable(() -> {
            contribution.setActualAmount(piggy.getActualAmount());
            return contributionRepository.insertWithCallback(contribution);
        });

        return updatePiggy
                .andThen(insertContribution);
    }