Spring Mongo reactive Handle Database Error on Save - project-reactor

Question

Can someone please help me as how do i handle database exception when i use Reactive Mongo with Spring webflux ...
I have a
Repository class
public interface UserRepository extends ReactiveMongoRepository<User,String>
{
public Mono<User> findByUserName(String userName);
}
Handler method in UserHandler
public Mono<ServerResponse> saveUser(ServerRequest request) {
Mono<User> user = request.bodyToMono(User.class).map(userObj -> {
userObj.setPassword(passwordEncoder.encode(userObj.getPassword()));
return userObj;
});
return
ServerResponse.ok().body(this.userRepository.insert(user),User.class);
}
I have defined a unique key on username so when the exception throw i want to return a meaningful message to use how can i use OnErrorMap to return a server reponse with message.
I get the below error in console but no error returned to the user
at sun.nio.ch.Invoker$2.run(Invoker.java:218) ~[na:1.8.0_60]
at sun.nio.ch.AsynchronousChannelGroupImpl$1.run(AsynchronousChannelGroupImpl.java:112) ~[na:1.8.0_60]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) ~[na:1.8.0_60]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) ~[na:1.8.0_60]
at java.lang.Thread.run(Thread.java:745) ~[na:1.8.0_60]
Caused by: com.mongodb.MongoWriteException: E11000 duplicate key error collection: letsbuy.users index: username dup key: { : "asoni11" }
at com.mongodb.async.client.MongoCollectionImpl$8.onResult(MongoCollectionImpl.java:638) ~[mongodb-driver-async-3.4.3.jar:na]
... 163 common frames omitted

Answer 1

It's a bit counterintuitive, but in that case you need to do something like:
Mono<User> savedUser = request.bodyToMono(User.class).map(userObj -> {
userObj.setPassword(passwordEncoder.encode(userObj.getPassword()));
return userObj;
})
.flatMap(user -> this.userRepository.insert(user));
return savedUser
.flatMap(u -> ServerResponse.ok().syncBody(u))
.onErrorResume(DuplicateKeyException.class,
t -> ServerResponse.status(HttpStatus.BAD_REQUEST).syncBody(t.getMessage()));