我一直在努力与链接表中的附加列建立多对多关系。
这些是我的实体:
@JsonIgnoreProperties({ "hibernateLazyInitializer", "handler" })
public class Post {
@Id
@GeneratedValue
private Long id;
private String name;
@OneToMany(mappedBy = "post", cascade = CascadeType.ALL, orphanRemoval = true)
@JsonIgnore
private List<PostTag> tags = new ArrayList<>();
//getters and setters
public void addTag(Tag tag){
PostTag postTag = new PostTag(this, tag);
tags.add(postTag);
tag.getPosts().add(postTag);
}
public void removeTag(Tag tag) {
for (Iterator<PostTag> iterator = tags.iterator();
iterator.hasNext(); ) {
PostTag postTag = iterator.next();
if (postTag.getPost().equals(this) &&
postTag.getTag().equals(tag)) {
iterator.remove();
postTag.getTag().getPosts().remove(postTag);
postTag.setPost(null);
postTag.setTag(null);
}
}
}
@Override
public boolean equals(Object o) {
if (this == o)
return true;
if (o == null || getClass() != o.getClass())
return false;
Post post = (Post) o;
return id == post.id;
}
@Override
public int hashCode() {
return Objects.hash(id);
}
}
@JsonIgnoreProperties({ "hibernateLazyInitializer", "handler" })
public class Tag {
@Id
@GeneratedValue
private Long id;
private String comment;
@OneToMany(mappedBy = "tag", cascade = CascadeType.ALL, orphanRemoval = true)
@JsonIgnore
private List<PostTag> posts = new ArrayList<>();
//getters and setters
@Override
public boolean equals(Object o) {
if (this == o)
return true;
if (o == null || getClass() != o.getClass())
return false;
Tag that = (Tag) o;
return id == that.id;
}
@Override
public int hashCode() {
return Objects.hash(id);
}
}
@Entity(name = "PostTag")
@Table(name = "post_tag")
@JsonIgnoreProperties({ "hibernateLazyInitializer", "handler" })
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;
private Integer impact;
public FacilityParticipant(Post post, Tag tag) {
this.post = post;
this.tag = tag;
this.id = new PostTagId(post.getId(), tag.getId());
}
//getters and setters
@Override
public boolean equals(Object o) {
if (this == o)
return true;
if (o == null || getClass() != o.getClass())
return false;
PostTag that = (PostTag) o;
return Objects.equals(post, that.post) && Objects.equals(tag, that.tag);
}
@Override
public int hashCode() {
return Objects.hash(post, tag);
}
}
@Embeddable
public class PostTagId implements Serializable {
private Long postId;
private Long tagId;
//getters setters
@Override
public boolean equals(Object o) {
if (this == o)
return true;
if (o == null || getClass() != o.getClass())
return false;
PostTagId that = (PostTagId) o;
return Objects.equals(postId, that.postId) && Objects.equals(tagId, that.tagId);
}
@Override
public int hashCode() {
return Objects.hash(postId, tagId);
}
}
我有一个映射到许多标签的帖子实体和一个映射到许多帖子的标签。链接表是 PostTag，它包含到两侧的映射，以及附加列“影响”。链接表的PK被映射到一个Embeddable表PostTagId，其中包含来自Post和Tag的PK。
当我尝试插入新实体时，我会执行以下操作:
Tag tag1 = new Tag();
Tag tag2 = new Tag();
repository.save(tag1);
repository.save(tag2);
Post post1 = new Post();
Post post2 = new Post();
post1.addTag(tag1);
post1.addTag(tag2);
post2.addTag(tag1);
repository.save(post1);
repository.save(post2);
尝试插入这些项目时，出现错误:无法将 NULL 插入 ("POST_TAG"."ID")
我尝试过的任何操作，要么会出现其他错误，要么会立即返回。
很可能模型中的某些内容不正确，但我真的无法弄清楚它出了什么问题。
整个建模是基于这篇文章The best way to ...
任何帮助将不胜感激。
谢谢
最佳答案
spring-data-jpa 是 JPA 之上的一层。每个实体都有自己的存储库，您必须处理它。我已经看过上面提到的教程，它是针对 JPA 的，并且还将 ID 设置为 null，这似乎有点不对劲，可能是导致错误的原因。我没看那么近。为了处理 spring-data-jpa 中的问题，您需要一个单独的链接表存储库。
@Entity
public class Post {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
@OneToMany(mappedBy = "post", cascade = CascadeType.ALL, orphanRemoval = true)
private List<PostTag> tags;
@Entity
public class Tag {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
@OneToMany(mappedBy = "tag", cascade = CascadeType.ALL, orphanRemoval = true)
private List<PostTag> posts;
@Entity
public class PostTag {
@EmbeddedId
private PostTagId id = new PostTagId();
@ManyToOne(fetch = FetchType.LAZY)
@MapsId("postId")
private Post post;
@ManyToOne(fetch = FetchType.LAZY)
@MapsId("tagId")
private Tag tag;
public PostTag() {}
public PostTag(Post post, Tag tag) {
this.post = post;
this.tag = tag;
}
@SuppressWarnings("serial")
@Embeddable
public class PostTagId implements Serializable {
private Long postId;
private Long tagId;
@Override
public boolean equals(Object o) {
if (this == o)
return true;
if (o == null || getClass() != o.getClass())
return false;
PostTagId that = (PostTagId) o;
return Objects.equals(postId, that.postId) && Objects.equals(tagId, that.tagId);
}
@Override
public int hashCode() {
return Objects.hash(postId, tagId);
}
并使用它，如上所示:
@Transactional
private void update() {
System.out.println("Step 1");
Tag tag1 = new Tag();
Post post1 = new Post();
PostTag p1t1 = new PostTag(post1, tag1);
tagRepo.save(tag1);
postRepo.save(post1);
postTagRepo.save(p1t1);
System.out.println("Step 2");
Tag tag2 = new Tag();
Post post2 = new Post();
PostTag p2t2 = new PostTag(post2, tag2);
postRepo.save(post2);
tagRepo.save(tag2);
postTagRepo.save(p2t2);
System.out.println("Step 3");
tag2 = tagRepo.getOneWithPosts(2L);
tag2.getPosts().add(new PostTag(post1, tag2));
tagRepo.save(tag2);
System.out.println("Step 4 -- better");
PostTag p2t1 = new PostTag(post2, tag1);
postTagRepo.save(p2t1);
}
请注意，有一些变化。我没有明确设置 PostTagId id。这些由持久层(在本例中为 hibernate 层)处理。
另请注意，您可以使用自己的存储库显式更新 PostTag 条目，也可以通过在设置了 CascadeType.ALL 后在列表中添加和删除它们来更新它们，如图所示。对 spring-data-jpa 使用 CascadeType.ALL 的问题是，即使您预取连接表实体，spring-data-jpa 仍然会再次执行此操作。尝试通过 CascadeType.ALL 更新新实体的关系是有问题的。
如果没有 CascadeType，posts 或 tags 列表(应该是 Sets)都不是关系的所有者，因此添加到它们不会在持久性方面没有完成任何事情，并且仅用于查询结果。
在读取 PostTag 关系时，您需要专门获取它们，因为您没有 FetchType.EAGER。如果您不需要联接，并且将其同时放在 Tag 和 Post 上，则 FetchType.EAGER 的问题是开销您将创建一个递归获取，获取任何查询的所有Tags 和Posts。
@Query("select t from Tag t left outer join fetch t.posts tps left outer join fetch tps.post where t.id = :id")
Tag getOneWithPosts(@Param("id") Long id);
最后，请务必检查日志。请注意，创建关联需要 spring-data-jpa (我认为是 JPA)读取现有表以查看关系是新的还是更新的。无论您自己创建并保存 PostTag 还是预取列表，都会发生这种情况。 JPA 有一个单独的合并，我认为您可以更有效地使用它。
create table post (id bigint generated by default as identity, primary key (id))
create table post_tag (post_id bigint not null, tag_id bigint not null, primary key (post_id, tag_id))
create table tag (id bigint generated by default as identity, primary key (id))
alter table post_tag add constraint FKc2auetuvsec0k566l0eyvr9cs foreign key (post_id) references post
alter table post_tag add constraint FKac1wdchd2pnur3fl225obmlg0 foreign key (tag_id) references tag
Step 1
insert into tag (id) values (null)
insert into post (id) values (null)
select posttag0_.post_id as post_id1_1_0_, posttag0_.tag_id as tag_id2_1_0_ from post_tag posttag0_ where posttag0_.post_id=? and posttag0_.tag_id=?
insert into post_tag (post_id, tag_id) values (?, ?)
Step 2
insert into post (id) values (null)
insert into tag (id) values (null)
select posttag0_.post_id as post_id1_1_0_, posttag0_.tag_id as tag_id2_1_0_ from post_tag posttag0_ where posttag0_.post_id=? and posttag0_.tag_id=?
insert into post_tag (post_id, tag_id) values (?, ?)
Step 3
select tag0_.id as id1_2_0_, posts1_.post_id as post_id1_1_1_, posts1_.tag_id as tag_id2_1_1_, post2_.id as id1_0_2_, posts1_.tag_id as tag_id2_1_0__, posts1_.post_id as post_id1_1_0__ from tag tag0_ left outer join post_tag posts1_ on tag0_.id=posts1_.tag_id left outer join post post2_ on posts1_.post_id=post2_.id where tag0_.id=?
select tag0_.id as id1_2_1_, posts1_.tag_id as tag_id2_1_3_, posts1_.post_id as post_id1_1_3_, posts1_.post_id as post_id1_1_0_, posts1_.tag_id as tag_id2_1_0_ from tag tag0_ left outer join post_tag posts1_ on tag0_.id=posts1_.tag_id where tag0_.id=?
select posttag0_.post_id as post_id1_1_0_, posttag0_.tag_id as tag_id2_1_0_ from post_tag posttag0_ where posttag0_.post_id=? and posttag0_.tag_id=?
insert into post_tag (post_id, tag_id) values (?, ?)
Step 4 -- better
select posttag0_.post_id as post_id1_1_0_, posttag0_.tag_id as tag_id2_1_0_ from post_tag posttag0_ where posttag0_.post_id=? and posttag0_.tag_id=?
insert into post_tag (post_id, tag_id) values (?, ?)
关于java - Spring-Data-JPA ManyToMany 关系与额外列，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/52648330/