Storing user only in mongo db if condition from webflux mono is true in kotlin - spring

Question

I have the following question. I want to save a user into a mongo db via reactive spring data repositories only if the e-mail of the user is not already present.
#Component("userService")
class UserService(private val repository: UserRepository){
fun checkIfEMailExists(email: String): Mono<Boolean> {
return repository.findByEMail(email).hasElement()
}
fun create(user: User): Mono<User> {
//not sure how to do this bit here
this.checkIfEMailExists(user.email)
.filter{ it -> it == true}
.map{repository.save(user)}
}
}
So, basically I am not sure how i can handle the Mono of boolean to do something only if it's value is true (and otherwise thrown an exception)

Answer 1

First, your MongoDB repository should be a reactive one and return a Mono<User> or Flux<User> for that kind of signature:
public interface UserRepository extends ReactiveMongoRepository<User, String> {
Mono<User> findByEmail(String email);
}
Then you can chain that reactive type with other operators like this:
#Service
public class UserService {
private final UserRepository userRepository;
public UserService(UserRepository userRepository) {
this.userRepository = userRepository;
}
public Mono<User> create(User user) {
return this.userRepository.findByEmail(user.getEmail())
.flatMap(existingUser -> Mono.error(new UserAlreadyPresentException(existingUser.getEmail())))
.then(this.userRepository.save(user));
}
class UserAlreadyPresentException extends RuntimeException {
public UserAlreadyPresentException(String email) {
super("User already present with email " + email);
}
}
}