ReactiveMongoRepository not saving my data

If you use reactive streams, you should be aware that nothing happens until you subscribe:

In Reactor, when you write a Publisher chain, data does not start pumping into it by default. Instead, you create an abstract description of your asynchronous process (which can help with reusability and composition).

By the act of subscribing, you tie the Publisher to a Subscriber, which triggers the flow of data in the whole chain. This is achieved internally by a single request signal from the Subscriber that is propagated upstream, all the way back to the source Publisher.

That means that if you use ReactiveMongoRepository, somewhere along the line you have to subscribe to your reactive stream. This can be done by using the subscribe() method on any Publisher. For example, using Java, that would be:

reactiveRepository    .save(myEntity)    .subscribe(result => log.info("Entity has been saved: {}", result));

Additionally, frameworks like Webflux will handle the subscription for you if you write reactive controllers. Using Java that means you could write something like:

@GetMappingpublic Mono<MyEntity> save() {    return reactiveRepository.save(myEntity); // subscribe() is done by the Webflux framework}

Obviously, there's a lot more to reactive programming than just that, and you should be aware that you cannot simply swap MongoRepository to ReactiveMongoRepository without adapting to the reactive ecosystem and using reactive programming.

There's an Introduction to Reactive Programming chapter within the Reactor reference guide that might be interesting to read. The earlier quoted documentation also comes from this chapter of the reference guide.

To make you stream terminal with subscribe() operation and to get the Mono result at the same time - split into two separate operations:

val a0 = myEntity(name = "my Entity");Mono<?> savedEntity = repository.save(a0);savedEntity.subscribe();return savedEntity;