orderService.java

    @Transactional
    public Optional<Order> placeOrder(Long productId, Long userId) {
        Optional<Product> product = productRepository.findById(productId);
        product.orElseThrow(() -> new RuntimeException("Invalid product id"));

        if (product.get().getAvailableUnits() > 0) {
            productRepository.decrementAvailableUnitsCountBy1(productId);
            Order newOrder = new Order(userId, productId);
            orderRepository.save(newOrder);
            return Optional.of(newOrder);
        }
        return Optional.empty();
    }