M : N(다대다) 연관관계 - 1

이전에 작성한 ERD를 보면 book와 author가 다대다 관계이다.

 

1. Author

---

  Author와 Book은 M : N 관계이다. books에 @ManyToMany를 적용

@Entity
@Data
@NoArgsConstructor
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper = true)
public class Author extends BaseEntity{

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long Id;

    private String name;
    private String country;

    @ManyToMany
    @ToString.Exclude
    private List<Book> books = new ArrayList<>();

    public void addBook(Book ... book){ //배열로 받겠다.
      Collections.addAll(this.books, book); //book 정보가 여러개 들어오면 한꺼번에 저장.
    }

}

* Repository

public interface AuthorRepository extends JpaRepository<Author, Long> {
}

 

 

2. Book

---

  Author와 Book은 M : N 관계이다. authors에 @ManyToMany를 적용

@Entity
@NoArgsConstructor
@Data
@EntityListeners(value = AuditingEntityListener.class)
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper = true)
public class Book extends BaseEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    private Long authorId;

    private String category;

    @OneToOne(mappedBy = "book")
    @ToString.Exclude
    private BookReviewInfo bookReviewInfo;

    @OneToMany
    @JoinColumn(name = "book_id") //중간 테이블 만들지 않기 위해, 나의pk를 상대의 fk로
    @ToString.Exclude
    private List<Review> reviews = new ArrayList<>(); //null point exception 방지

    @ManyToOne
    @ToString.Exclude
    private Publisher publisher;

    @ManyToMany
    @ToString.Exclude
    private List<Author> authors = new ArrayList<>(); //NPE방지하기위해 array생성

    public void addAuthor(Author ... author){

        Collections.addAll(this.authors, author);

    }

}

* Collections???

 

3. table 생성 확인

---

 

author_books라는 table과 book_authors라는 table이 생성되었다.

Hibernate: 
    
    create table author (
       id bigint generated by default as identity,
        created_at timestamp,
        updated_at timestamp,
        country varchar(255),
        name varchar(255),
        primary key (id)
    )
    
    
Hibernate: 
    
    create table author_books (
       author_id bigint not null,
        books_id bigint not null
    )
    
    
Hibernate: 
    
    create table book (
       id bigint generated by default as identity,
        created_at timestamp,
        updated_at timestamp,
        author_id bigint,
        category varchar(255),
        name varchar(255),
        publisher_id bigint,
        primary key (id)
    )
    
    Hibernate: 
    
    create table book_authors (
       book_id bigint not null,
        authors_id bigint not null
    )

one to Many에서는 중간 table을 @JoinColumn(name = "")으로 제거를 했고,

One의 PK를 FK로 Many에서 가지고 있었다.

 

그러면 many to many처럼 one 이 없으면? FK로 삼을 PK를 어디서구함?

 

그래서 중간 table을 구성하고 여기에서 book의 id와 authors의 id를 맵핑한 것이다.

 

4. Test

@SpringBootTest
class AuthorRepositoryTest {

    @Autowired
    private AuthorRepository authorRepository;

    @Autowired
    private BookRepository bookRepository;

    @Test
    @Transactional
    void manyToManyTest(){

        Book book1 = givenBook("책1");
        Book book2 = givenBook("책2");
        Book book3 = givenBook("개발책1");
        Book book4 = givenBook("개발책2");

        Author author1 = givenAuthor("martin");
        Author author2 = givenAuthor("steve");

        book1.addAuthor(author1);
        book2.addAuthor(author2);
        book3.addAuthor(author1,author2);
        book4.addAuthor(author1, author2);

        author1.addBook(book1, book3, book4);
        author2.addBook(book2, book3, book4);

        //여기까지 연관관계 다 맺음

        bookRepository.saveAll(Lists.newArrayList(book1, book2, book3, book4)); //save all은 lists를 받아서 다저장해줌
        authorRepository.saveAll(Lists.newArrayList(author1, author2));

        System.out.println("authors through book : " + bookRepository.findAll().get(2).getAuthors());
        System.out.println("books through author : " + authorRepository.findAll().get(0).getBooks());


    }


    private Book givenBook(String name){
        Book book  =  new Book();
        book.setName(name);

        return bookRepository.save(book);
    }

    private Author givenAuthor(String name){
        Author author = new Author();
        author.setName(name);

        return authorRepository.save(author);
    }



}

 

 

 

5. 결과

---

book에서 authors를, author에서 books를 출력할 수 있다.