천천히, 꾸준히, 어제보다 나은 현재의 나를 만들자

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M:N 연관관계-중간 테이블을 엔티티로 관리하는 경우

https://raw.githubusercontent.com/hy6219/TIL/main/Spring/JPA/Entity/RelationShip/OneToOne.png

TIL/[Spring JPA]M to N 연관관계-중간 테이블을 엔티티로 관리하지 않는 경우.md at main · hy6219/TIL

지난 시간에 살짝 언급했듯이, “ManyToMany”에서는 중간 테이블을 JoinColumn 등으로 해소할 수가 없다

이번 시간에 해보려고 하는 것을 예로 들자면 “주문 테이블”이 그 사례가 될 수 있을 것 같다

주문에서 ManyToMany관계에 해당되는 것은 바로 “사용자”와 “상품”간의 관계인데

이렇게 되면 저번 시간에 살펴본 것처럼 user_product라는 중간테이블이 생겨나게 될 것이다

이 때, 이 “user_product”라는 테이블을 따로 “order”라는 엔티티 클래스로 관리하기도 하는데 , 이번에는 이렇게 중간 테이블을 클래스로 관리하는 방식을 접해보도록 할 것이다

01. 중간 테이블을 위한 클래스 “BookAndAuthor” 만들기

  • BookAndAuthor: Book=N: 1
  • BookAndAuthor: Author=N:1
package com.example.jpa_entity.domain;

import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.NoArgsConstructor;
import lombok.ToString;

import javax.persistence.*;

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

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

    @ManyToOne
    private Book book;

    @ManyToOne
    private Author author;

}

02. 01 과정에 따라서, Book과 BookAndAuthor, BookAndAuthor 와 Author관계 성립시켜주기

이제는 중간 브릿지로써 “BookAndAuthor”가 존재해서

  • Book:BookAndAuthor=1:N
  • Author:BookAndAuthor=1:N

으로 양측을 정리해줄 필요가 있다

그렇게 되면 @JoinColumn(name="book-id") 로 중간테이블과 연결될 수 있다!

package com.example.jpa_entity.domain;

import com.example.jpa_entity.domain.listener.Auditable;
import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.NoArgsConstructor;
import lombok.ToString;
import org.springframework.data.annotation.CreatedDate;
import org.springframework.data.annotation.LastModifiedDate;
import org.springframework.data.jpa.domain.support.AuditingEntityListener;

import javax.persistence.*;
import java.time.LocalDateTime;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

@Data
@NoArgsConstructor
@Entity
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper=true)
///@EntityListeners(value=MyEntityListener.class)
//@EntityListeners(value= AuditingEntityListener.class)
public class Book extends BaseEntity/* implements Auditable*/ {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    //책 이름
    private String name;

    private String category;

    //책 저자
    private Long authorId;

//    private Long publisherId;

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

    @OneToMany
    @JoinColumn(name="book_id")
    @ToString.Exclude
    private List<Review> reviews=new ArrayList<>();

    @ManyToOne
    @ToString.Exclude
    private Publisher publisher;

    //@ManyToMany
    **@OneToMany
    @JoinColumn(name="book_id")
    @ToString.Exclude
    private List<BookAndAuthor> bookAndAuthors=new ArrayList<>();**

//    public void addAuthor(Author...author){
//        Collections.addAll(this.authors,author);
//    }

}

package com.example.jpa_entity.domain;

import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.NoArgsConstructor;
import lombok.ToString;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

@Data
@NoArgsConstructor
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper = true)
@Entity
public class Author extends BaseEntity{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private String country;

  //  @ManyToMany
    **@OneToMany
    @JoinColumn(name="author_id")
    @ToString.Exclude
    private List<BookAndAuthor> bookAndAuthors=new ArrayList<>();**

//    public void addBook(Book...book){
//        Collections.addAll(this.books,book);
//    }

}

03. BookAndAuthor(중간 테이블)에 대한 레포지토리 생성

package com.example.jpa_entity.repository;

import com.example.jpa_entity.domain.BookAndAuthor;
import org.springframework.data.jpa.repository.JpaRepository;

public interface BookAndAuthorRepository extends JpaRepository<BookAndAuthor,Long> {
}

04. 테스트 코드에서 연관관계를 맺는 부분을 수정

03까지 진행되었다면, 연관관계를 맺는 부분을 수정해줄 필요가 있다

package com.example.jpa_entity.repository;

import com.example.jpa_entity.domain.Author;
import com.example.jpa_entity.domain.Book;
import com.example.jpa_entity.domain.BookAndAuthor;
import org.assertj.core.util.Lists;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import javax.transaction.Transactional;
import java.util.ArrayList;
import java.util.Arrays;

import static org.junit.jupiter.api.Assertions.*;

@SpringBootTest
class AuthorRepositoryTest {

    @Autowired
    private AuthorRepository authorRepository;

    @Autowired
    private BookRepository bookRepository;

    @Autowired
    private BookAndAuthorRepository bookAndAuthorRepository;

    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);
    }

    private BookAndAuthor givenBookAndAuthor(Book book, Author author){
        BookAndAuthor ba=new BookAndAuthor();
        ba.setBook(book);
        ba.setAuthor(author);

        return bookAndAuthorRepository.save(ba);
    }

    @Test
    @Transactional
    public 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");

        //중간테이블 엔티티
        BookAndAuthor ba1=givenBookAndAuthor(book1,author1);
        BookAndAuthor ba2=givenBookAndAuthor(book2,author2);
        BookAndAuthor ba3=givenBookAndAuthor(book3,author1);
        BookAndAuthor ba4=givenBookAndAuthor(book3,author2);
        BookAndAuthor ba5=givenBookAndAuthor(book4,author1);
        BookAndAuthor ba6=givenBookAndAuthor(book4,author2);

        book1.setBookAndAuthors(Lists.newArrayList(ba1));
        book2.setBookAndAuthors(Lists.newArrayList(ba2));
        book3.setBookAndAuthors(Lists.newArrayList(ba3,ba4));
        book4.setBookAndAuthors(Lists.newArrayList(ba5,ba6));

        author1.setBookAndAuthors(Lists.newArrayList(ba1,ba3,ba5));
        author2.setBookAndAuthors(Lists.newArrayList(ba2,ba4,ba6));

        bookRepository.saveAll(Lists.newArrayList(book1,book2,book3,book4));
        authorRepository.saveAll(Lists.newArrayList(author1,author2));

        //연관관계 넣어주기
//        book1.addAuthor(author1);
//        book2.addAuthor(author2);
//        book3.addAuthor(author1,author2);
//        book4.addAuthor(author1,author2);
//
//        author1.addBook(book1,book3,book4);
//        author2.addBook(book2,book3,book4);

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

        /*
        * 주문 테이블에서는 User와 Product가 M:N관계가 될 수 있을것
        * ->따라서 User,Product 테이블 외에도
        * 중간 테이블로써 user_products가 ddl에서 보이게 될 것
        * 그런데 이 중간 테이블을 또다른 엔티티인 "order테이블"로써
        * 따로 엔티티를 만들어줄 수도 있을 것
        * */

    }
}

이렇게 Book 객체와 Author 객체를 인자로 받아서 객체를 만들고 모두 저장을 해준 후에 확인을 해도 좋지만 보다 좋은 방식을 위해서 5의 방식을 도입해보자

05. 보다 현업에서 사용되는 방식으로 중간 엔티티 객체 관리하기

저번에 addXXX로 데이터를 추가하는 것에 대한 관리방법으로 중간 엔티티도 동일하게 접근해보자

package com.example.jpa_entity.domain;

import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.NoArgsConstructor;
import lombok.ToString;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

@Data
@NoArgsConstructor
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper = true)
@Entity
public class Author extends BaseEntity{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private String country;

  //  @ManyToMany
    @OneToMany
    @JoinColumn(name="author_id")
    @ToString.Exclude
    private List<BookAndAuthor> bookAndAuthors=new ArrayList<>();

//    public void addBook(Book...book){
//        Collections.addAll(this.books,book);
//    }

    **public void addBookAndAuthor(BookAndAuthor...bookAndAuthor){
        Collections.addAll(this.bookAndAuthors,bookAndAuthor);
    }**
}

package com.example.jpa_entity.domain;

import com.example.jpa_entity.domain.listener.Auditable;
import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.NoArgsConstructor;
import lombok.ToString;
import org.springframework.data.annotation.CreatedDate;
import org.springframework.data.annotation.LastModifiedDate;
import org.springframework.data.jpa.domain.support.AuditingEntityListener;

import javax.persistence.*;
import java.time.LocalDateTime;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

@Data
@NoArgsConstructor
@Entity
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper=true)
///@EntityListeners(value=MyEntityListener.class)
//@EntityListeners(value= AuditingEntityListener.class)
public class Book extends BaseEntity/* implements Auditable*/ {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    //책 이름
    private String name;

    private String category;

    //책 저자
    private Long authorId;

//    private Long publisherId;

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

    @OneToMany
    @JoinColumn(name="book_id")
    @ToString.Exclude
    private List<Review> reviews=new ArrayList<>();

    @ManyToOne
    @ToString.Exclude
    private Publisher publisher;

    //@ManyToMany
    @OneToMany
    @JoinColumn(name="book_id")
    @ToString.Exclude
    private List<BookAndAuthor> bookAndAuthors=new ArrayList<>();

//    public void addAuthor(Author...author){
//        Collections.addAll(this.authors,author);
//    }

    **public void addBookAndAuthor(BookAndAuthor...bookAndAuthor){
        Collections.addAll(this.bookAndAuthors,bookAndAuthor);
    }**

}

package com.example.jpa_entity.repository;

import com.example.jpa_entity.domain.Author;
import com.example.jpa_entity.domain.Book;
import com.example.jpa_entity.domain.BookAndAuthor;
import org.assertj.core.util.Lists;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import javax.transaction.Transactional;
import java.util.ArrayList;
import java.util.Arrays;

import static org.junit.jupiter.api.Assertions.*;

@SpringBootTest
class AuthorRepositoryTest {

    @Autowired
    private AuthorRepository authorRepository;

    @Autowired
    private BookRepository bookRepository;

    @Autowired
    private BookAndAuthorRepository bookAndAuthorRepository;

    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);
    }

    private BookAndAuthor givenBookAndAuthor(Book book, Author author){
        BookAndAuthor ba=new BookAndAuthor();
        ba.setBook(book);
        ba.setAuthor(author);

        return bookAndAuthorRepository.save(ba);
    }

    @Test
    @Transactional
    public 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");

        //중간테이블 엔티티
        BookAndAuthor ba1=givenBookAndAuthor(book1,author1);
        BookAndAuthor ba2=givenBookAndAuthor(book2,author2);
        BookAndAuthor ba3=givenBookAndAuthor(book3,author1);
        BookAndAuthor ba4=givenBookAndAuthor(book3,author2);
        BookAndAuthor ba5=givenBookAndAuthor(book4,author1);
        BookAndAuthor ba6=givenBookAndAuthor(book4,author2);

        **book1.addBookAndAuthor(ba1);
        book2.addBookAndAuthor(ba2);
        book3.addBookAndAuthor(ba3,ba4);
        book4.addBookAndAuthor(ba5,ba6);

        author1.addBookAndAuthor(ba1,ba3,ba5);
        author2.addBookAndAuthor(ba2,ba4,ba6);**
//        book1.setBookAndAuthors(Lists.newArrayList(ba1));
//        book2.setBookAndAuthors(Lists.newArrayList(ba2));
//        book3.setBookAndAuthors(Lists.newArrayList(ba3,ba4));
//        book4.setBookAndAuthors(Lists.newArrayList(ba5,ba6));
//
//        author1.setBookAndAuthors(Lists.newArrayList(ba1,ba3,ba5));
//        author2.setBookAndAuthors(Lists.newArrayList(ba2,ba4,ba6));

        bookRepository.saveAll(Lists.newArrayList(book1,book2,book3,book4));
        authorRepository.saveAll(Lists.newArrayList(author1,author2));

				System.out.printf("get book and author info from book table--\n");
        bookRepository.findAll().get(2).getBookAndAuthors().forEach(item->{
            System.out.printf("book: %s, author: %s\n",item.getBook(),item.getAuthor());
        });

        //연관관계 넣어주기
//        book1.addAuthor(author1);
//        book2.addAuthor(author2);
//        book3.addAuthor(author1,author2);
//        book4.addAuthor(author1,author2);
//
//        author1.addBook(book1,book3,book4);
//        author2.addBook(book2,book3,book4);

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

        /*
        * 주문 테이블에서는 User와 Product가 M:N관계가 될 수 있을것
        * ->따라서 User,Product 테이블 외에도
        * 중간 테이블로써 user_products가 ddl에서 보이게 될 것
        * 그런데 이 중간 테이블을 또다른 엔티티인 "order테이블"로써
        * 따로 엔티티를 만들어줄 수도 있을 것
        * */

    }
}

그러면 테스트코드는 더이상 setter로 일일히 추가해줄 필요가 없다

Hibernate: 
    insert 
    into
        book
        (id, created_at, updated_at, author_id, category, name, publisher_id) 
    values
        (null, ?, ?, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book
        (id, created_at, updated_at, author_id, category, name, publisher_id) 
    values
        (null, ?, ?, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book
        (id, created_at, updated_at, author_id, category, name, publisher_id) 
    values
        (null, ?, ?, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book
        (id, created_at, updated_at, author_id, category, name, publisher_id) 
    values
        (null, ?, ?, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        author
        (id, created_at, updated_at, country, name) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        author
        (id, created_at, updated_at, country, name) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
get book and author info from book_and_author table--
Hibernate: 
    select
        book0_.id as id1_2_,
        book0_.created_at as created_2_2_,
        book0_.updated_at as updated_3_2_,
        book0_.author_id as author_i4_2_,
        book0_.category as category5_2_,
        book0_.name as name6_2_,
        book0_.publisher_id as publishe7_2_ 
    from
        book book0_
book: Book(super=BaseEntity(cratedAt=2021-08-31T16:45:33.133, updatedAt=2021-08-31T16:45:33.133), id=3, name=개발책1, category=null, authorId=null), author: Author(super=BaseEntity(cratedAt=2021-08-31T16:45:33.144, updatedAt=2021-08-31T16:45:33.144), id=1, name=martin, country=null)
book: Book(super=BaseEntity(cratedAt=2021-08-31T16:45:33.133, updatedAt=2021-08-31T16:45:33.133), id=3, name=개발책1, category=null, authorId=null), author: Author(super=BaseEntity(cratedAt=2021-08-31T16:45:33.150, updatedAt=2021-08-31T16:45:33.150), id=2, name=steve, country=null)

지금은 book3에 대해서 접근하기 때문에, 책 이름은 당연히 같고

저자는 지금 author1과 author2가 들어가 있는 것을 확인해볼 수 있다

마찬가지로, Author 테이블에서도 동일과정으로 작업을 진행해볼 수 있다

이번에는 author1로 접근해서, 저자는 같지만 book1,book3,book4의 정보를 확인해볼 수 있게 된다

package com.example.jpa_entity.repository;

import com.example.jpa_entity.domain.Author;
import com.example.jpa_entity.domain.Book;
import com.example.jpa_entity.domain.BookAndAuthor;
import org.assertj.core.util.Lists;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import javax.transaction.Transactional;
import java.util.ArrayList;
import java.util.Arrays;

import static org.junit.jupiter.api.Assertions.*;

@SpringBootTest
class AuthorRepositoryTest {

    @Autowired
    private AuthorRepository authorRepository;

    @Autowired
    private BookRepository bookRepository;

    @Autowired
    private BookAndAuthorRepository bookAndAuthorRepository;

    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);
    }

    private BookAndAuthor givenBookAndAuthor(Book book, Author author){
        BookAndAuthor ba=new BookAndAuthor();
        ba.setBook(book);
        ba.setAuthor(author);

        return bookAndAuthorRepository.save(ba);
    }

    @Test
    @Transactional
    public 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");

        //중간테이블 엔티티
        BookAndAuthor ba1=givenBookAndAuthor(book1,author1);
        BookAndAuthor ba2=givenBookAndAuthor(book2,author2);
        BookAndAuthor ba3=givenBookAndAuthor(book3,author1);
        BookAndAuthor ba4=givenBookAndAuthor(book3,author2);
        BookAndAuthor ba5=givenBookAndAuthor(book4,author1);
        BookAndAuthor ba6=givenBookAndAuthor(book4,author2);

        book1.addBookAndAuthor(ba1);
        book2.addBookAndAuthor(ba2);
        book3.addBookAndAuthor(ba3,ba4);
        book4.addBookAndAuthor(ba5,ba6);

        author1.addBookAndAuthor(ba1,ba3,ba5);
        author2.addBookAndAuthor(ba2,ba4,ba6);
//        book1.setBookAndAuthors(Lists.newArrayList(ba1));
//        book2.setBookAndAuthors(Lists.newArrayList(ba2));
//        book3.setBookAndAuthors(Lists.newArrayList(ba3,ba4));
//        book4.setBookAndAuthors(Lists.newArrayList(ba5,ba6));
//
//        author1.setBookAndAuthors(Lists.newArrayList(ba1,ba3,ba5));
//        author2.setBookAndAuthors(Lists.newArrayList(ba2,ba4,ba6));

        bookRepository.saveAll(Lists.newArrayList(book1,book2,book3,book4));
        authorRepository.saveAll(Lists.newArrayList(author1,author2));

        System.out.printf("get book and author info from book table--\n");
        bookRepository.findAll().get(2).getBookAndAuthors().forEach(item->{
            System.out.printf("book: %s, author: %s\n",item.getBook(),item.getAuthor());
        });

        System.out.printf("get book and author info from author table--\n");
        **authorRepository.findAll().get(0).getBookAndAuthors().forEach(item->{
            System.out.printf("book: %s, author: %s\n",item.getBook(),item.getAuthor());
        });**

        //연관관계 넣어주기
//        book1.addAuthor(author1);
//        book2.addAuthor(author2);
//        book3.addAuthor(author1,author2);
//        book4.addAuthor(author1,author2);
//
//        author1.addBook(book1,book3,book4);
//        author2.addBook(book2,book3,book4);

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

        /*
        * 주문 테이블에서는 User와 Product가 M:N관계가 될 수 있을것
        * ->따라서 User,Product 테이블 외에도
        * 중간 테이블로써 user_products가 ddl에서 보이게 될 것
        * 그런데 이 중간 테이블을 또다른 엔티티인 "order테이블"로써
        * 따로 엔티티를 만들어줄 수도 있을 것
        * */

    }
}

Hibernate: 
    insert 
    into
        book
        (id, created_at, updated_at, author_id, category, name, publisher_id) 
    values
        (null, ?, ?, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book
        (id, created_at, updated_at, author_id, category, name, publisher_id) 
    values
        (null, ?, ?, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book
        (id, created_at, updated_at, author_id, category, name, publisher_id) 
    values
        (null, ?, ?, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book
        (id, created_at, updated_at, author_id, category, name, publisher_id) 
    values
        (null, ?, ?, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        author
        (id, created_at, updated_at, country, name) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        author
        (id, created_at, updated_at, country, name) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
Hibernate: 
    insert 
    into
        book_and_author
        (id, created_at, updated_at, author_id, book_id) 
    values
        (null, ?, ?, ?, ?)
get book and author info from book table--
Hibernate: 
    select
        book0_.id as id1_2_,
        book0_.created_at as created_2_2_,
        book0_.updated_at as updated_3_2_,
        book0_.author_id as author_i4_2_,
        book0_.category as category5_2_,
        book0_.name as name6_2_,
        book0_.publisher_id as publishe7_2_ 
    from
        book book0_
book: Book(super=BaseEntity(cratedAt=2021-08-31T16:50:51.854, updatedAt=2021-08-31T16:50:51.854), id=3, name=개발책1, category=null, authorId=null), author: Author(super=BaseEntity(cratedAt=2021-08-31T16:50:51.864, updatedAt=2021-08-31T16:50:51.864), id=1, name=martin, country=null)
book: Book(super=BaseEntity(cratedAt=2021-08-31T16:50:51.854, updatedAt=2021-08-31T16:50:51.854), id=3, name=개발책1, category=null, authorId=null), author: Author(super=BaseEntity(cratedAt=2021-08-31T16:50:51.872, updatedAt=2021-08-31T16:50:51.872), id=2, name=steve, country=null)
get book and author info from author table--
Hibernate: 
    select
        author0_.id as id1_1_,
        author0_.created_at as created_2_1_,
        author0_.updated_at as updated_3_1_,
        author0_.country as country4_1_,
        author0_.name as name5_1_ 
    from
        author author0_
**book: Book(super=BaseEntity(cratedAt=2021-08-31T16:50:51.762, updatedAt=2021-08-31T16:50:51.762), id=1, name=책1, category=null, authorId=null), author: Author(super=BaseEntity(cratedAt=2021-08-31T16:50:51.864, updatedAt=2021-08-31T16:50:51.864), id=1, name=martin, country=null)
book: Book(super=BaseEntity(cratedAt=2021-08-31T16:50:51.854, updatedAt=2021-08-31T16:50:51.854), id=3, name=개발책1, category=null, authorId=null), author: Author(super=BaseEntity(cratedAt=2021-08-31T16:50:51.864, updatedAt=2021-08-31T16:50:51.864), id=1, name=martin, country=null)
book: Book(super=BaseEntity(cratedAt=2021-08-31T16:50:51.856, updatedAt=2021-08-31T16:50:51.856), id=4, name=개발책2, category=null, authorId=null), author: Author(super=BaseEntity(cratedAt=2021-08-31T16:50:51.864, updatedAt=2021-08-31T16:50:51.864), id=1, name=martin, country=null)**

그리고 DDL을 살펴보면, 중간 테이블에 이전에 author_id와 book_id에 붙었던 not null이 없어졌고,

created_at, updated_at, pk가 보이는 것을 확인해볼 수 있었다

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 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_and_author (
       id bigint generated by default as identity,
        created_at timestamp,
        updated_at timestamp,
        author_id bigint,
        book_id bigint,
        primary key (id)
    )**

Updated:

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