Spring WebFlux check user exists

Question

I want to check that the user has not been created yet before creating a new one, if there is then create an error... I found a similar question, but I can't remake it =(

Spring WebFlux: Emit exception upon null value in Spring Data MongoDB reactive repositories?

  public Mono<CustomerDto> createCustomer(Mono<CustomerDto> dtoMono) {
    //How Create Mono error???
    Mono<Customer> fallback = Mono.error(new DealBoardException("Customer with email: " + dtoMono ???));

    return dtoMono.map(customerConverter::convertDto) //convert from DTO to Document
        .map(document -> {
          customerRepository.findByEmailOrPhone(document.getEmail(), document.getPhone())
        })
        .switchIfEmpty() //How check such customer doesn't exists?
        .map(document -> { //Filling in additional information from other services
          var customerRequest = customerConverter.convertDocumentToStripe(document);
          var customerStripe = customerExternalService.createCustomer(customerRequest);
          document.setCustomerId(customerStripe.getId());
          return document;
        })
        .flatMap(customerRepository::save) //Save to MongoDB
        .map(customerConverter::convertDocument); //convert from Document to Dto
  }

Answer 1

Add the following index declaration on the top of your Customer class:

@CompoundIndex(def = "{'email': 1, 'phone': 1}", unique = true)

That will prevent duplicate entries to be inserted in the database.

You can catch your org.springframework.dao.DuplicateKeyException with the following construct:

customerService.save(newCustomer).onErrorMap(...);
        

Ref: MongoDB Compound Indexes official documentation