@Test
void test_books() {

    test_authors();
    Iterable<BookWithAuthors> books = bookWithAuthorRepository.saveAll(
            Stream.of("ololo", "trololo")
                  .map(title -> new BookWithAuthors(null, title))
                  .map(bookWithAuthors -> bookWithAuthors.addAll(authorOfBookRepository.findAll()))
                  .collect(Collectors.toList())
    );
    log.info("created books: {}", books);
    log.info("find all: {}", bookWithAuthorRepository.findAll());
}