我有一个应用程序，它基本上具有类似 Post/Comment 的功能。我需要应用程序从 Post 实体呈现 Post 索引页面，并从 Comment 实体显示最新评论和作者。
我的数据库/表结构是（id 是主要的，只是没有这样显示）和示例数据：
create table post
(
id int auto_increment,
title text not null,
constraint post_id_uindex
unique (id)
);
id,title
1,This is test post number 1
4,This is test post number 2
5,This is test post number 3
create table comment
(
id int auto_increment,
comment text null,
author text null,
created_at datetime null,
post_id int null,
constraint comment_id_uindex
unique (id),
constraint comment_post_id_fk
foreign key (post_id) references post (id)
);
id,comment,author,created_at,post_id
1,Nice Post this is,Stephen,2021-08-03 16:49:22,1
2,Another nice post,Billy,2021-08-03 16:49:44,1
3,I didn't like the post at all,Karen,2021-08-04 10:57:05,1
我在两个实体中设置了双向映射：
@Entity
public class Comment {
private int id;
private String comment;
private String author;
private Timestamp createdAt;
private Post postByPostId;
@Id
@Column(name = "id")
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
@Basic
@Column(name = "comment")
public String getComment() {
return comment;
}
public void setComment(String comment) {
this.comment = comment;
}
@Basic
@Column(name = "author")
public String getAuthor() {
return author;
}
public void setAuthor(String author) {
this.author = author;
}
@Basic
@Column(name = "created_at")
public Timestamp getCreatedAt() {
return createdAt;
}
public void setCreatedAt(Timestamp createdAt) {
this.createdAt = createdAt;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Comment comment1 = (Comment) o;
return id == comment1.id && Objects.equals(comment, comment1.comment) && Objects.equals(author, comment1.author) && Objects.equals(createdAt, comment1.createdAt);
}
@Override
public int hashCode() {
return Objects.hash(id, comment, author, createdAt);
}
@ManyToOne
@JoinColumn(name = "post_id", referencedColumnName = "id")
public Post getPostByPostId() {
return postByPostId;
}
public void setPostByPostId(Post postByPostId) {
this.postByPostId = postByPostId;
}
}
@Entity
public class Post {
private int id;
private String title;
private Collection<Comment> commentsById;
@Id
@Column(name = "id")
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
@Basic
@Column(name = "title")
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
@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 && Objects.equals(title, post.title);
}
@Override
public int hashCode() {
return Objects.hash(id, title);
}
@OneToMany(mappedBy = "postByPostId")
public Collection<Comment> getCommentsById() {
return commentsById;
}
public void setCommentsById(Collection<Comment> commentsById) {
this.commentsById = commentsById;
}
}
我正在使用的 PostRepository @Query 如下。查询本身作为原始 SQL 工作，我收到带有最新评论数据的帖子：
id,title,id,comment,author,created_at,post_id
1,This is test post number 1,3,I didn't like the post at all,Karen,2021-08-04 10:57:05,1
4,This is test post number 2,,,,,
5,This is test post number 3,,,,,
但是当我尝试使用 ${post.commentsById} 访问我的 Thymeleaf 模板中的评论字段时，所有评论都被显示，我希望只显示从 @Query 命令加入的一个评论对象。我还看到，当我删除 @Query 时，它会做同样的事情，在这种情况下使 @Query 毫无意义。
谁能给我指导如何正确设置我们？提前致谢。
@Repository
public interface PostRepository extends JpaRepository<Post, Long> {
@Query(value = "select p.*, c.* from post p left join (select a.* from comment a join (select max(created_at) created_at from comment group by post_id) b on a.created_at = b.created_at) c on p.id = c.post_id", nativeQuery = true)
List<Post> findAll();
}
post.html Thymeleaf 模板
<!DOCTYPE html>
<html lang="en" xmlns="http://www.w3.org/1999/html" xmlns:th="http://www.thymeleaf.org">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Posts</title>
</head>
<body>
<header>
<div class="text-center m-5">
<h1 class="display-6">Posts
<small class="text-muted" th:text="${title}"></small>
</h1>
</div>
</header>
<main>
<table>
<thead>
<tr>
<th>Post ID</th>
<th>Post Title</th>
<th>Latest Comment ID</th>
<th>Latest Comment</th>
<th>Author</th>
<th>Created At</th>
</tr>
</thead>
<tbody>
<tr th:each="post : ${posts}">
<td th:text="${post.getId} ?: '-'">-</td>
<td th:text="${post.title} ?: '-'">-</td>
<td th:text="${post.commentsById} ?: '-'"></td>
</tr>
</tbody>
</table>
</main>
</body>
</html>
使用 Comment 对象数组而不是来自 @Query 的最新评论呈现 HTML
Post ID Post Title Latest Comment ID Latest Comment Author Created At
1 This is test post number 1 [com.example.demo.entity.Comment@6d578e15, com.example.demo.entity.Comment@e74e80e, com.example.demo.entity.Comment@42b0db5]
4 This is test post number 2 []
5 This is test post number 3 []