I'm working on a project where I need to do CRUD operations on Book and Library objects. Naturally the relationship between Book and Library is Many to One, like so:
@Entity @Getter @Setter @AllArgsConstructor @NoArgsConstructor public class LibraryDao { @Id @GeneratedValue Long id; @OneToMany(mappedBy = "library") List<BookDao> book = new ArrayList<>(); } @Entity @Getter @Setter @AllArgsConstructor @NoArgsConstructor public class BookDao { @Id @GeneratedValue private long id; @ManyToOne(fetch = FetchType.LAZY) private LibraryDao library; String name; public BookDao(Library library, String name) { this.library = library; this.name = name; } } I'm trying to keep these as separate as possible in each layer which is where my question comes from. It's easy to keep the creation and getting of Library stuff separate, but I'm finding it hard to create persist Books without mixing the two up. Mainly because I need a LibraryDAO object to create a BookDAO. Please see my comment in the BookServiceImpl class.
@Service public class LibraryServiceImpl implements LibraryService { @Autowired LibraryRepository libraryRepository; @Override public Library getLibrary(long libraryId) { LibraryDao libraryDao = libraryRepository.findById(libraryId).orElseThrow(LibraryNotFoundException::new); return new Library(libraryDao.getId(), libraryDao.getBooks().stream().map(book -> new Book(book.getId(), book.getName())).collect(Collectors.toList())); } @Override public Library createLibrary() { LibraryDao libraryDao = libraryRepository.save(new LibraryDao()); return new Library(libraryDao.getId(), List.of()); } } @Service public class BookServiceImpl implements BookService { @Autowired BookRepository bookRepository; @Autowired LibraryService libraryService; @Override public Book createBook(Long libraryId, String name) { //Which one below is less destructive to the MVC pattern? Either I can //have a LibraryRepository in my BookService class (this feels wrong) //or else make libraryService.getLibrary() return a DAO object? Isn't is expected //that DAO objects stay in their service class? LibraryDao libraryDao = libraryService.getLibrary(libraryId); LibraryDao libraryDao = libraryRepository.findById(libraryId); BookDao bookDao = new BookDao(libraryDao, name); BookingDao saved = bookRepository.save(bookDao); return new Book(saved.getId(), saved.getLibraryDao().getId(), saved.getName()); } 
