How to check if Mono is empty?

Question

I\'m developing a app with Spring Boot 2.0 and Kotlin using the WebFlux framework.

I want to check if a user id exits before save a transaction. I\'m stucked in a simple

Answer 1

Let me start by saying I am a newbie on reactive (java) and on this forum. I think you cannot really check in this code if a mono is empty because a mono represents code that will be executed later on, so in this code body you won't know yet if its is empty. Does that make sense?

I just wrote something similar in Java which seems to work (but not 100% this is the best approach either):

    public Mono queryStore(ServerRequest request) { 

        Optional postalCode = request.queryParam("postalCode");                            

        Mono badQuery = ServerResponse.badRequest().build();
        Mono notFound = ServerResponse.notFound().build();

        if (!postalCode.isPresent()) { return  badQuery; }

        Flux stores = this.repository
                .getNearByStores(postalCode.get(), 5);

        return ServerResponse.ok().contentType(APPLICATION_JSON)
                .body(stores, Store.class)
                .switchIfEmpty(notFound);
}