JPA ManToMany 关联并使用 Spring Data Repository 插入、检索数据

Question

我需要在 2 个实体（即项目和文章）之间设置多对多关系。 用例是一个可以链接到许多相关文章的项目。每篇文章都会链接到各种项目。 我在我的项目中使用 Spring 数据存储库，并且无法持久保存和检索与项目和文章的这种关系。

@Entity @Table(name = "projects")
@Getter @Setter
public class Project {
    @Id
    String id;
    String name;

    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinTable(name = "project_articles",
        joinColumns = { @JoinColumn(name = "project_id", referencedColumnName = "id") },
        inverseJoinColumns = { @JoinColumn(name = "article_id", referencedColumnName = "id") })
    private List<Article> articles = new ArrayList<>();

    public Project addArticle(Article p) {
        if (articles == null) articles = new ArrayList<>();
        p.getProjects().add(this);
        articles.add(p);
        return this;
    }
}

文章实体

@Entity @Table(name = "articles")
@Getter @Setter
public class Article {

    @Id
    String id;
    String title;
    String author;
    String body;

    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinTable(name = "project_articles")
    List<Project> projects = new ArrayList<>();

}

ProjectArticles 连接表实体。我创建它是为了解决休眠中的表创建问题。

@Entity(name = "ProjectArticles")
@Table(name = "project_articles")
@AssociationOverrides({
    @AssociationOverride(name = "key.project", joinColumns = @JoinColumn(name = "project_id")),
    @AssociationOverride(name = "key.article", joinColumns = @JoinColumn(name = "article_id")) }
)
@Getter @Setter
public class ProjectArticles {
    @EmbeddedId ProjectArticleId key;

    @Embeddable @Getter @Setter
    public static class ProjectArticleId {
        @Column(name = "project_id", columnDefinition = "varchar(255)")
        ProjectEntity project;

        @Column(name = "article_id", columnDefinition = "varchar(255)")
        Article article;
    }
}

这里是 spring 数据存储库。

public interface ProjectRepository extends JpaRepository<Project, String> {
    public Project findProjectByName(String name);
    public List<Project> findProjectByArticle(String articleId);
}

public interface ArticleRepository extends JpaRepository<Article, String> {
    public List<Article> findArticlesByTitle(String title);

    // NOTE: Does not work
    @Query("SELECT a from Article a JOIN FETCH a.projects LEFT JOIN ProjectEntity p ON p.id = :project")
    public List<Article> findArticlesByProject(@Param("project") String project);

    @Query("SELECT a from Article a LEFT JOIN ProjectEntity p ON p.id = :project")
    public List<Article> findArticleByProject(ProjectEntity project);
}

理想情况下，我想在服务中使用 repo，如下所示。

Article ar1 = articleRepository.save(new Article("Article title", "Author 1"));
Article ar2 = articleRepository.save(new Article("Article title 2", "Author 2"));
Article ar3 = articleRepository.save(new Article("Article title 3", "Author 3"));

Project pr1 = projectRepository.save(new Project("Project name 1"));
Project pr2 = projectRepository.save(new Project("Project name 2"));
Project pr3 = projectRepository.save(new Project("Project name 3"));

pr1.addArticle(ar1);
pr1.addArticle(ar2);
pr2.addArticle(ar1);
pr2.addArticle(ar2);

ar3.addProject(pr3);
ar3.addProject(pr1);
ar3.addProject(pr2);

articleRepository.findArticlesByProject("project-2-id");
projectRepository.findProjectByArticle("article-3-id");

但事情不工作。当我将现有文章添加到项目并保存（更新）它时，在关系表（“project_articles”）中尝试输入新行但没有项目 ID，并且出现异常。

我在这里做错了什么？我设计实体的方式有可能吗？

Answer 1

CascadeType.PERSIST（你使用的CascadeType.ALL涉及）的主要思想是创建实体实例，设置必要的双边关系（Project -> Articles，Article ->项目），然后才保存到数据库。如果您保存所有项目，那么由于CascadeType.PERSIST 和实体关系，文章会被隐式保存

试试这个

@Transactional
public void save() {
    Article ar1 = new Article("Article title", "Author 1");
    Article ar2 = new Article("Article title 2", "Author 2");
    Article ar3 = new Article("Article title 3", "Author 3");

    Project pr1 = new Project("Project name 1");
    Project pr2 = new Project("Project name 2");
    Project pr3 = new Project("Project name 3");

    pr1.addArticle(ar1);
    pr1.addArticle(ar2);
    pr1.addArticle(ar3);

    pr2.addArticle(ar1);
    pr2.addArticle(ar2);
    pr2.addArticle(ar3);

    pr3.addArticle(ar3);

    projectRepository.saveAll(Arrays.asList(pr1, pr2, pr3));
}

Answer 2

我已按照几篇博文中描述的指南解决了这种情况。

• Introduction to Spring Data JPA Part 8: Many-to-Many Bidirectional
• JPA / Hibernate Many to Many Mapping Example with Spring Boot

另外，我需要解决 JSON 序列化无限递归问题。

这是最终的 JPA 模型。

@Entity @Table(name = "projects")
@Getter @Setter
public class Project {
    @Id
    String id;
    String name;

    @JsonIgnoreProperties("projects")
    @ManyToMany(cascade = { PERSIST, MERGE, DETACH, REFRESH }, fetch = FetchType.LAZY)
    @JoinTable(name = "project_articles",
        joinColumns = { @JoinColumn(name = "project_id", referencedColumnName = "id") },
        inverseJoinColumns = { @JoinColumn(name = "article_id", referencedColumnName = "id") })
    private Set<Article> articles = new HashSet<>();

    public Project addArticle(Article art) {
        articles.add(art);
        art.getProjects().add(this);
        return this;
    }
}

更新了文章实体。

@Entity @Table(name = "articles")
@Getter @Setter
public class Article {

    @Id
    String id;
    String title;
    String author;
    String body;

    @JsonIgnoreProperties("articles")
    @ManyToMany(mappedBy = "articles", cascade = { PERSIST, MERGE, DETACH, REFRESH }, fetch = FetchType.LAZY)
    Set<Project> projects = new HashSet<>();

    public Article addProject(Project prj) {
        projects.add(prj);
        prj.getArticles().add(this);
        return this;
    }    
}

解决了实体保存/更新问题。

Article ar1 = articleRepository.save(new Article("Article title", "Author 1"));
Article ar2 = articleRepository.save(new Article("Article title 2", "Author 2"));
Article ar3 = articleRepository.save(new Article("Article title 3", "Author 3"));

Project pr1 = projectRepository.save(new Project("Project name 1"));
Project pr2 = projectRepository.save(new Project("Project name 2"));
Project pr3 = projectRepository.save(new Project("Project name 3"));

/* Link articles 1 and 3 to project 1 */
pr1.getArticles().add(ar1);
pr1.getArticles().add(ar3);
ar1.getProjects().add(pr1);
ar3.getProjects().add(pr1);

/* Link articles 2 and 3 to project 2 */
pr2.addArticle(ar3);
pr2.addArticle(ar2);

/* Link project 3 to article 3 */
ar3.addProject(pr3);

但尚未解决数据获取问题。例如获取链接到一个项目的所有项目。 （待办事项）