java - 从另一个角度提问

Question

我的项目中有四个实体: 主题、文章、评论和通知。

One topic can have multiple articles.

One article can have multiple comments.

You can add multiple notifications for comment.

这些是我的实体:

@Entity
@Table(name = "topics")
public class Topic {

    @Id
    private Integer id;
    private String name;
    private String description;
    @OneToMany(mappedBy = "article")
    private List<Article> articles;
    //getters & setters ommited
}


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

    @Id
    private Integer id;
    private String name;
    private String description;
    @ManyToOne
    @JoinColumn(name = "topic_id")
    private Topic topic;
    @OneToMany(mappedBy = "comment")
    private List<Comment> comments;
    //getters & setters ommited
}

@Entity
@Table(name = "comments")
public class Comment {
    @Id
    private Integer id;
    private String name;
    private String description;
    private LocalDateTime createDate;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "article_id")
    private Article article;
    //getters & setters ommited
}

最后一个:

@Entity
@Table(name = "notifications")
public class Notification {
    @Id
    private Integer id;
    private LocalDate date;
    private String description;
    @ManyToOne(optional = false)
    @JoinColumn(name = "comment_id", nullable = false)
    private Comment comment;
    //getters & setters ommited
}

现在我试图实现的是获取一组在日期之间带有通知的主题。

我什至创建了一个解决方案:

public Set<Topic> getTopicsWithNotificationsBetweenDates(LocalDate begin, LocalDate end) {
    return notificationRepository.findByDateBetween(begin, end)
            .stream()
            .map(notification-> getTopic(notification)))
            .collect(Collectors.toSet());
}

private Topic getTopic(Notification notification){
    return notification.getComment().getArticle().getTopic();
}

但是这个解决方案循环遍历所有通知来获取主题(显然有重复)。从客户那里获取它们会节省大量时间和精力，以防万一。 100 条通知 并且仅例如5 个主题。

现在我想做的是循环遍历主题而不是通知，但我不知道查询应该是什么样子。 即使是一点点帮助，或者至少指出正确的方向，我都会感激不尽。

Answer 1

在 Comment 和 Notification 实体之间添加双向关系怎么样？然后您将能够在单个查询中执行您想要的操作，如下所示:

List<Topic> findByArticlesCommentsNotificationsDateBetween(begin, end);