Maybe<Post> findById(UUID id) {
        return findAll().filter(p -> p.getId().equals(id))
                .singleElement()
                .switchIfEmpty(Maybe.error(new PostNotFoundException(id)));
    }
@Test
    void findById() {
        var testObserver = new TestObserver<Post>();
        this.posts.findById(UUID.randomUUID()).subscribe(testObserver);

        testObserver.assertError(PostNotFoundException.class);
        testObserver.assertNotComplete();
    }