51工具盒子简介 {#简介}
本文将带你了解，当无法依赖 CascadeType 机制来将状态转换从父实体传播到子实体时，如何使用 Spring Data JPA 级联删除单向关联，
Domain Model {#domain-model}
假设我们的系统中有以下实体：
Post 实体是该实体层次结构的根（root），子实体只使用单向 @ManyToOne 或 @OneToOne 关联，用于映射一个一对多或一对一表关系的底层外键。
Post root 实体如下：
@Entity @Table(name = "post") public class Post {@Id private Long id;private String title;
// getter/setter 方法省略
}
请注意，没有双向的 @OneToMany 或 @OneToOne 关联，可以允许我们从父级 Post 级联 DELETE 操作到 PostComment、PostDetails 或 PostTag 子实体。
PostComment 实体使用单向 @ManyToOne 关联映射 post_id 外键列：
@Entity @Table(name = "post_comment") public class PostComment {@Id @GeneratedValue private Long id;@ManyToOne(fetch = FetchType.LAZY) private Post post;
private String review;
//getter/setter 方法省略
}
PostDetails 实体使用单向 @OneToOne 关联映射 id 外键列：
@Entity @Table(name = "post_details") public class PostDetails {@Id private Long id;@OneToOne(fetch = FetchType.LAZY) @MapsId private Post post;
@Column(name = "created_on") private LocalDateTime createdOn;
@Column(name = "created_by") private String createdBy;
//getter/setter 方法省略
}
PostTag 实体使用单向 @ManyToOne 关联映射 post_id 外键列：
@Entity @Table(name = "post_tag") public class PostTag {@EmbeddedId private PostTagId id;@ManyToOne(fetch = FetchType.LAZY) @MapsId("postId") private Post post;
@ManyToOne(fetch = FetchType.LAZY) @MapsId("tagId") private Tag tag;
@Column(name = "created_on") private Date createdOn = new Date();
//getter/setter 方法省略
}
由于 UserVote 实体是 PostComment 的子实体，因此 Post 实体不仅有直接的子关联。因此，它是 Post root 实体的孙子关联：
@Entity @Table(name = "user_vote") public class UserVote {@Id @GeneratedValue private Long id;@ManyToOne(fetch = FetchType.LAZY) private User user;
@ManyToOne(fetch = FetchType.LAZY) private PostComment comment;
private int score;
//getter/setter 方法省略
}
创建 Post 实体 {#创建-post-实体}
创建一个包含以下内容的 Post 实体：
- 一个
PostDetails子实体 - 两个
PostComment子实体，每个子实体都有一个UserVote实体 - 三个
PostTag子实体
Post post = new Post() .setId(1L) .setTitle("High-Performance Java Persistence"); postRepository.persist(post);postDetailsRepository.persist( new PostDetails() .setCreatedBy("Vlad Mihalcea") .setPost(post) );
PostComment comment1 = new PostComment() .setReview("Best book on JPA and Hibernate!") .setPost(post);
PostComment comment2 = new PostComment() .setReview("A must-read for every Java developer!") .setPost(post);
postCommentRepository.persist(comment1); postCommentRepository.persist(comment2);
User alice = new User() .setId(1L) .setName("Alice");
User bob = new User() .setId(2L) .setName("Bob");
userRepository.persist(alice); userRepository.persist(bob);
userVoteRepository.persist( new UserVote() .setUser(alice) .setComment(comment1) .setScore(Math.random() > 0.5 ? 1 : -1) );
userVoteRepository.persist( new UserVote() .setUser(bob) .setComment(comment2) .setScore(Math.random() > 0.5 ? 1 : -1) );
Tag jdbc = new Tag().setName("JDBC"); Tag hibernate = new Tag().setName("Hibernate"); Tag jOOQ = new Tag().setName("jOOQ");
tagRepository.persist(jdbc); tagRepository.persist(hibernate); tagRepository.persist(jOOQ);
postTagRepository.persist(new PostTag(post, jdbc)); postTagRepository.persist(new PostTag(post, hibernate)); postTagRepository.persist(new PostTag(post, jOOQ));
如何使用 Spring Data JPA 级联 DELETE 单向关联？ {#如何使用-spring-data-jpa-级联-delete-单向关联}
现在，我们希望有一种方法来移除给定的 Post 实体，如果我们在之前创建的 Post 实体上使用 PostRepository 的默认 deleteById 方法：
postRepository.deleteById(1L);
将会抛出 ConstraintViolationException：
Caused by: org.postgresql.util.PSQLException:
ERROR:
update or delete on table "post" violates
foreign key constraint "fk_post_comment_post_id"
on table "post_comment"
Detail:
Key (id)=(1) is still referenced
from table "post_comment".
抛出 ConstraintViolationException 的原因是 post 表记录仍被 post_details、post_comment 和 post_tag 表中的子实体引用。
因此，我们需要确保在删除某个 Post 实体之前，先删除所有子实体。
为此，修改 PostRepository，继承 CustomPostRepository 接口：
@Repository public interface PostRepository extends BaseJpaRepository<Post, Long>, CustomPostRepository<Long> {
}
CustomPostRepository 定义了 deleteById 方法，我们打算在自定义 JPA Repository 中覆写该方法，这样就可以将 DELETE 操作从 Post 实体级联到所有单向关联：
public interface CustomPostRepository<ID> {void deleteById(ID postId);
}
有关自定义 Spring Data JPA Repository 的介绍，可以参考 官方文档。
CustomPostRepository 接口如下：
public class CustomPostRepositoryImpl implements CustomPostRepository<Long> {private final PostDetailsRepository postDetailsRepository;private final UserVoteRepository userVoteRepository;
private final PostCommentRepository postCommentRepository;
private final PostTagRepository postTagRepository;
private final EntityManager entityManager;
public CustomPostRepositoryImpl( PostDetailsRepository postDetailsRepository, UserVoteRepository userVoteRepository, PostCommentRepository postCommentRepository, PostTagRepository postTagRepository, EntityManager entityManager) { this.postDetailsRepository = postDetailsRepository; this.userVoteRepository = userVoteRepository; this.postCommentRepository = postCommentRepository; this.postTagRepository = postTagRepository; this.entityManager = entityManager; }
@Override public void deleteById(Long postId) { Post post = null; PostDetails postDetails = postDetailsRepository.findWithPostById(postId);
if(postDetails != null) { postDetailsRepository.delete(postDetails); post = postDetails.getPost(); } else { post = entityManager.getReference(Post.class, postId); } userVoteRepository.deleteAllByPostId(postId); postCommentRepository.deleteAllByPostId(postId); postTagRepository.deleteAllByPostId(postId); entityManager.remove(post);
}
}
deleteById 方法的实现方式是，我们可以清理所有指向 Post 实体的关联子表记录，无论是直接通过外键还是间接通过一系列外键引用，就像 user_vote 表记录的情况一样。
因此，首先加载相关的 PostDetails，如果找到这个子实体，删除它并获取对 Post 实体的引用。如果找不到子实体，就加载一个 Post 实体引用，在最后一个操作中使用它来删除这个实体。
第二步，通过批量 DELETE 语句调用 UserVoteRepository 中的 deleteAllByPostId 方法来删除 UserVotes：
@Repository public interface UserVoteRepository extends BaseJpaRepository<UserVote, Long> {@Query(""" delete from UserVote where comment.id in ( select id from PostComment where post.id = :postId ) """) @Modifying void deleteAllByPostId(@Param("postId") Long postId);
}
接下来，使用单个批量 DELETE 语句调用 PostCommentRepository 中的 deleteAllByPostId 方法，删除 PostComment 子实体：
@Repository public interface PostCommentRepository extends BaseJpaRepository<PostComment, Long> {@Query(""" delete from PostComment where post.id = :postId """) @Modifying void deleteAllByPostId(@Param("postId") Long postId);
}
之后，使用批量 DELETE 语句调用 PostTagRepository 中的 deleteAllByPostId 方法，删除 PostTag 子实体：
@Repository public interface PostTagRepository extends BaseJpaRepository<PostTag, PostTagId> {@Query(""" delete from PostTag where post.id = :postId """) @Modifying void deleteAllByPostId(@Param("postId") Long postId);
}
最后，在删除所有子单向关联后，就可以继续删除 root Post 实体了。
当调用 PostRepository 上的 deleteById 方法时:
postRepository.deleteById(1L);
Spring Data JPA 和 Hibernate 将执行以下 SQL 语句：
SELECT pd.post_id, pd.created_by, pd.created_on, p.id, p.title FROM post_details pd JOIN post p ON p.id = pd.post_id WHERE pd.post_id = 1DELETE FROM user_vote WHERE comment_id IN ( SELECT pd.id FROM post_comment pd WHERE pd.post_id = 1 )
DELETE FROM post_comment WHERE post_id = 1
DELETE FROM post_tag WHERE post_id = 1
DELETE FROM post_details WHERE post_id = 1
DELETE FROM post WHERE id = 1
参考：https://vladmihalcea.com/cascade-delete-unidirectional-associations-spring/
