Issue with reactive nested flatMap calls - spring-webflux

Question

I need to perform an operation on two Monos. The difficulty is that one depend on the result of the other.
Let me explain:
I have a Mono<User> (I get that from a ServerRequest; User is a POJO).
I need to be able to extract the user email from the above Mono and pass it to the UserRepository in order to check whether the email already exists in DB.
If the user already exists I will throw a 400 error; otherwise, I will save the user contained in the ServerRequest.
Here is what I have tried in my handler:
public Mono<ServerResponse> saveUser(ServerRequest serverRequest) {
return serverRequest.bodyToMono(User.class)
.flatMap(user -> userRepository
.findByEmail(user.getEmail())
.flatMap(foundUser -> {
if (foundUser != null) {
System.out.println("found:" + foundUser);
throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "Email already exists");
} else {
System.out.println("creating" + user);
return status(CREATED).contentType(APPLICATION_JSON).body(userRepository.save(user), User.class);
}
}));
}
User:
#Data
#AllArgsConstructor
#NoArgsConstructor
public class User {
#Id
private Integer id;
#Size(min = 2)
private String firstName;
#Size(min = 2)
private String lastName;
#Email
private String email;
}
UserRepository:
public interface UserRepository extends ReactiveCrudRepository<User, Long> {
#Query("select id, first_name, last_name, email from user u where u.email = :email")
Mono<User> findByEmail(String email);
}
However, It seems there's an issue with my subscription: none of my System.out.println are called when the endpoint is called. Can someone please help?
edit 1: Here is the router calling the above handler method:
#Configuration
public class UserRouter {
#Bean
public RouterFunction<ServerResponse> route(UserHandler userHandler) {
return RouterFunctions.route()
.GET("/api/user", accept(APPLICATION_JSON), userHandler::getUsers)
.POST("/api/sign-up", accept(APPLICATION_JSON), userHandler::saveUser)
.build();
}
}

Answer 1

The issue here is that you expect a null when the user is not found, however, in reactive streams null is invalid. Instead, reactive streams have a dedicated empty state and dedicated operators to handle the empty case.
In your example you could do the following:
public Mono<ServerResponse> saveUser(ServerRequest serverRequest)
{
return serverRequest.bodyToMono(User.class)
.flatMap(this::createUserIfNotExists);
}
private Mono<ServerResponse> createUserIfNotExists(User user)
{
return userRepository.findByEmail(user.getEmail())
.hasElement()
.flatMap(foundUser ->
{
if (foundUser)
{
System.out.println("found user");
throw new ResponseStatusException(HttpStatus.BAD_REQUEST,
"Email already exists");
} else
{
System.out.println("creating user");
return status(CREATED).contentType(APPLICATION_JSON)
.body(userRepository.save(user), User.class);
}
});
}
or another alternative:
public Mono<ServerResponse> saveUser(ServerRequest serverRequest)
{
return serverRequest.bodyToMono(User.class)
.flatMap(this::createUserIfNotExists);
}
private Mono<ServerResponse> createUserIfNotExists(User user)
{
return userRepository.findByEmail(user.getEmail())
.flatMap(foundUser ->
status(BAD_REQUEST)
.contentType(APPLICATION_JSON)
.body(BodyInserters.fromObject("User already exists."))
)
.switchIfEmpty(
userRepository.save(user)
.flatMap(newUser -> status(CREATED).contentType(APPLICATION_JSON)
.body(BodyInserters.fromObject(newUser)))
);
}