@Getter
@Setter
@Entity
@NoArgsConstructor
@Table(name = "post")
public class Post extends DateAudit {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "post_id")
    private Long postId;

    private String title;
		...(생략)...
     
    @Lob
    private String content;
		...(생략)...
}

@Getter
@Setter
@Entity
@NoArgsConstructor
@Table(name = "comment")
public class Comment extends DateAudit {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "comment_id")
    private Long commentId;
		...(생략)...

    //연관관계 매팽
    @ManyToOne
    @JoinColumn(name = "post_id")
    private Post post;
  	...(생략)...
}

@Slf4j
@RunWith(SpringRunner.class)
@DataJpaTest
public class PostRepositoryTest {
    @Autowired
    private PostRepository postRepository;

    @Test
    public void save_post_확인() {
        postRepository.save(Post.builder()
                .title("title1")
                .author("frank")
                .likeCount(5)
                .content("content").build());
        List<Post> posts = postRepository.findAll();
        assertThat(posts.get(0).getTitle()).isEqualTo("title1");
    }
}

@Slf4j
@RunWith(SpringRunner.class)
@DataJpaTest
public class CommentRepositoryTest {
    @Autowired
    private CommentRepository commentRepository;

    @Autowired
    private PostRepository postRepository;

    @Test
    public void save_post_comment_확인() {
        Post post = postRepository.save(Post.builder()
                .title("title")
                .author("postAuthor")
                .likeCount(5)
                .build());

        Comment comment = Comment.builder()
                .author("frank")
                .content("content").build();
        comment.setPost(post);

        commentRepository.save(comment);
        List<Comment> comments = commentRepository.findAll();

        assertThat(comments.get(0).getAuthor()).isEqualTo("frank");
        assertThat(comments.get(0).getPost().getAuthor()).isEqualTo("postAuthor");
    }
}

@ToString(exclude = "post")
@Entity
@Table(name = "comment")
public class Comment extends DateAudit {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "comment_id")
    private Long commentId;
		...(생략)...

    //연관관계 매팽
    @ManyToOne
    @JoinColumn(name = "post_id", nullable = false)
    private Post post; //연관관계의 주인이 된다
		...(생략)...
}

@Entity
@Table(name = "post")
public class Post extends DateAudit {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "post_id")
    private Long postId;
  	...(생략)...

    //양방향 연관관계 설정
    @JsonIgnore //JSON 변환시 무한 루프 방지용
    @OneToMany(mappedBy = "post", fetch = FetchType.LAZY)
    private List<Comment> comments = new ArrayList<>();
		...(생략)...
}

comment.setPost(post); //(1) 코멘트 -> 포스트
post.getComments().add(comment); //(2) 포스트 -> 코멘트 - 저장시 사용되지 않는다

public class Comment extends DateAudit {
	public void setPost(Post post) {
    if (this.post != null) { //기존 포스트 관계를 제거함
			this.post.getComments().remove(this);
    }
    this.post = post;
    post.getComments().add(this);
  }
}

public class Comment extends DateAudit {
  	...(생략)...
    public void setPost(Post post) {
        this.post = post;
        //무한루프에 빠지지 않도록 체크
        if (!post.getComments().contains(this)) {
            post.getComments().add(this);
        }
    }
}

public class Post extends DateAudit {
    ...(생략)...
    public void addComment(Comment comment) {
        this.comments.add(comment);
        //무한루프에 빠지지 않도록 체크
        if (comment.getPost() != this) {
            comment.setPost(this);
        }
    }
}

@Table(name = "comment")
public class Comment extends DateAudit {
		...(생략)...   
    @ManyToOne
    @JoinColumn(name = "post_id")
    private Post post;
}

@Test
public void save_post_comment_확인_eager_loading() throws JsonProcessingException {
  Post post = postRepository.save(Post.builder()
                                  .title("title")
                                  .author("postAuthor")
                                  .likeCount(5)
                                  .build());

  Comment comment = Comment.builder()
    .author("frank")
    .content("content").build();
  comment.setPost(post);

  commentRepository.save(comment);

  Comment foundComment = commentRepository.findById(1L).get(); //이때 JOIN해서 Post 객체도 가져온다.
  assertThat(foundComment.getAuthor()).isEqualTo("frank");

  assertThat(foundComment.getPost().getAuthor()).isEqualTo("postAuthor");
}

select comment0_.comment_id as comment_1_0_0_, comment0_.create_dt as create_d2_0_0_, comment0_.updated_dt as updated_3_0_0_, comment0_.author as author4_0_0_, comment0_.content as content5_0_0_, comment0_.post_id as post_id6_0_0_, post1_.post_id as post_id1_1_1_, post1_.create_dt as create_d2_1_1_, post1_.updated_dt as updated_3_1_1_, post1_.author as author4_1_1_, post1_.content as content5_1_1_, post1_.like_count as like_cou6_1_1_, post1_.title as title7_1_1_ from comment comment0_ inner join post post1_ on comment0_.post_id=post1_.post_id where comment0_.comment_id=?

@Table(name = "comment")
public class Comment extends DateAudit {
		...(생략)...   
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "post_id")
    private Post post;
}

@Transactional
@Test
public void save_post_comment_확인_lazy_loading_test() throws JsonProcessingException {
  Post post = postRepository.save(Post.builder()
                                  .title("title")
                                  .author("postAuthor")
                                  .likeCount(5)
                                  .build());

  Comment comment = Comment.builder()
    .author("frank")
    .content("content").build();
  comment.setPost(post);

  commentRepository.save(comment);

  Comment foundComment = commentRepository.findById(1L).get();
  assertThat(foundComment.getAuthor()).isEqualTo("frank");

  Post foundPost = foundComment.getPost(); //(1) 객체 그래프 탐색
  String author = foundPost.getAuthor(); //(2) 팀 객체 실제 사용
  assertThat(author).isEqualTo("postAuthor");
}

@Table(name = "comment")
public class Comment extends DateAudit {
		...(생략)...   
    @ManyToOne
    @JoinColumn(name = "post_id")
    private Post post;
}

Comment foundComment = commentRepository.findById(1L).get();

Hibernate: select comment0_.comment_id as comment_1_0_0_, comment0_.create_dt as create_d2_0_0_, comment0_.updated_dt as updated_3_0_0_, comment0_.author as author4_0_0_, comment0_.content as content5_0_0_, comment0_.post_id as post_id6_0_0_, post1_.post_id as post_id1_1_1_, post1_.create_dt as create_d2_1_1_, post1_.updated_dt as updated_3_1_1_, post1_.author as author4_1_1_, post1_.content as content5_1_1_, post1_.like_count as like_cou6_1_1_, post1_.title as title7_1_1_ from comment comment0_ left outer join post post1_ on comment0_.post_id=post1_.post_id where comment0_.comment_id=?

@Table(name = "comment")
public class Comment extends DateAudit {
		...(생략)...   
    @ManyToOne(optional=false)
    @JoinColumn(name = "post_id")
    private Post post;
}

select comment0_.comment_id as comment_1_0_0_, comment0_.create_dt as create_d2_0_0_, comment0_.updated_dt as updated_3_0_0_, comment0_.author as author4_0_0_, comment0_.content as content5_0_0_, comment0_.post_id as post_id6_0_0_, post1_.post_id as post_id1_1_1_, post1_.create_dt as create_d2_1_1_, post1_.updated_dt as updated_3_1_1_, post1_.author as author4_1_1_, post1_.content as content5_1_1_, post1_.like_count as like_cou6_1_1_, post1_.title as title7_1_1_ from comment comment0_ inner join post post1_ on comment0_.post_id=post1_.post_id where comment0_.comment_id=?