WebFlux check field in DB

Question

Tell me how to properly organize the check in the Mongo collection so that it is not null

createStripCustomerId - Here I have a check, if the field is not filled, execute the request and save. If the field is filled, then just return the current object

It confuses me that customerRepository.save(customer) - returns Mono As a result, you have to wrap the customer in Mono

Maybe there is an elegant solution?

  private Mono<String> getStripCustomerId() {
    return currentUser.getCustomer()
        .map(customerLogin -> customerRepository.findById(customerLogin.getId()))
        .flatMap(this::createStripCustomerId)
        .map(Customer::getExternalCustomers)
        .map(map -> map.get(PaymentSystem.STRIPE));
  }

  private Mono<Customer> createStripCustomerId(Mono<Customer> customerMono) {
    return customerMono.flatMap(customer -> {
      if (customer.getExternalCustomers()
          .get(PaymentSystem.STRIPE) == null) {
        var request = customerConverter.convertDocumentToStripe(customer);
        var response = customerExternalService.createCustomer(request);
        customer.setExternalCustomers(Map.of(PaymentSystem.STRIPE, response.getCustomerId()));
        return customerRepository.save(customer);
      } else {
        return Mono.fromCallable(() -> customer); - This wrapper..
      }
    });
  }