@Override
    public Mono<FavouriteProduct> findFavouriteProductByProduct(int productId, String userId) {
        return this.favouriteProductRepository.findByProductIdAndUserId(productId, userId);
    }
@Test
    void findFavouriteProductByProduct_ReturnsFavouriteProduct() {
        // given
        doReturn(Mono.just(new FavouriteProduct(UUID.fromString("fe87eef6-cbd7-11ee-aeb6-275dac91de02"),
                1, "5f1d5cf8-cbd6-11ee-9579-cf24d050b47c")))
                .when(this.favouriteProductRepository)
                .findByProductIdAndUserId(1, "5f1d5cf8-cbd6-11ee-9579-cf24d050b47c");

        // when
        StepVerifier.create(this.service.findFavouriteProductByProduct(1, "5f1d5cf8-cbd6-11ee-9579-cf24d050b47c"))
                // then
                .expectNext(new FavouriteProduct(UUID.fromString("fe87eef6-cbd7-11ee-aeb6-275dac91de02"),
                        1, "5f1d5cf8-cbd6-11ee-9579-cf24d050b47c"))
                .verifyComplete();
    }