@Entity
@Data
@Table(name = "books")
public class Book {

    @JsonBackReference
    @ManyToMany(mappedBy = "books")
    private Set<Author> authors;

}
@Table(name = "authors")
@Data
@Entity
public class Author {

    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
    @JoinTable(name = "author_book",
                joinColumns = @JoinColumn(name = "author_id", referencedColumnName = "author_id"),
                inverseJoinColumns = @JoinColumn(name = "book_id", referencedColumnName = "book_id"))
    @JsonManagedReference
    private Set<Book> books;

}
@Slf4j
@Service
@AllArgsConstructor
public class AuthorService {
    private final AuthorRepo authorRepo;
    private final BookRepo bookRepo;

    public Author findById(Long id) {
        Author author = authorRepo.findById(id)
                .orElseThrow(() -> new ResourceNotFoundException("Author not found with id: " + id));

        Hibernate.initialize(author.getBooks());

        return author;
    }

    public Author addBookToAuthor(Long authorId, Long bookId) {
        Author author = authorRepo.findById(authorId)
                .orElseThrow(() -> new ResourceNotFoundException("Author not found with id: " + authorId));
        Book book = bookRepo.findById(bookId)
                .orElseThrow(() -> new ResourceNotFoundException("Book not found with id: " + bookId));

        Set<Book> books = author.getBooks();
        if (books == null) {
            books = new HashSet<>();
        }

        books.add(book);
        author.setBooks(books);

        author = authorRepo.save(author);
        authorRepo.flush();

        return author;
    }

}
@RestController
@RequestMapping("authors")
@AllArgsConstructor
public class AuthorController {
    private final AuthorService authorService;


    @ResponseStatus(HttpStatus.OK)
    @GetMapping("/{id}")
    public Author findAuthorById(@PathVariable Long id) {
        return authorService.findById(id);
    }

    @ResponseStatus(HttpStatus.OK)
    @PutMapping("/{authorId}/AddBook")
    public Author addBookToAuthor(@PathVariable Long authorId, Long bookId) {
        return authorService.addBookToAuthor(authorId, bookId);
    }


}