Aggregate sub-entities in query results using JPA and QueryDSL

php editor Baicao brought an article about using JPA and QueryDSL to aggregate sub-entities in query results. In this article, we will delve into how to handle the query results of sub-entities when performing aggregate queries using JPA and QueryDSL. By studying this article, readers will be able to understand how to implement aggregate queries on sub-entities through JPA and QueryDSL, and obtain accurate and efficient query results. Whether you are a beginner or an experienced developer, this article will provide you with valuable knowledge and tips to help you better utilize JPA and QueryDSL to handle sub-entities in aggregate queries in real projects.

Question content

I am developing a java application using jpa and querydsl and facing the challenge of one-to-many relationship query. I have three entities: Articles, Comments and Reactions. Each post (one) can have multiple comments and reactions (many). I need to get each article and its aggregated comments and reactions.

This is my current approach:

public page<articledetail> findarticles(pagerequest pagerequest, user currentuser) {
    var articles = new jpaqueryfactory(entitymanager)
            .select(projections.constructor(articledetail.class,
                    article.id,
                    projections.constructor(userdetail.class,
                            user.id,
                            user.name,
                            user.username,
                            user.email,
                            user.profilepicture,
                            user.level,
                            user.position),
                    article.content,
                    article.type,
                    projections.list(projections.constructor(commentdetail.class,
                            comment.user.id,
                            comment.article.id,
                            comment.text,
                            comment.timestamp).skipnulls()).skipnulls(),
                    projections.list(projections.constructor(reactiondetail.class,
                            reaction.user.id,
                            reaction.type).skipnulls()).skipnulls(),
                    article.commentcount,
                    article.datecreated,
                    article.datelastmodified
                ))
                .from(article)
                .innerjoin(article.user, user)
                .leftjoin(article.comments, comment).on(comment.isactive.istrue())
                .leftjoin(article.reactions, reaction)
                .where(article.isactive.istrue(),
                        user.status.eq(status.active),
                        article.user.in(currentuser.getfollowing())
                                .or(article.user.eq(currentuser)))
                .offset(pagerequest.getoffset())
                .limit(pagerequest.getpagesize())
                .orderby(article.id.asc())
                .fetch();

    return new pageimpl<>(articles, pagerequest, articles.size());
}

entity:

@Entity
public class Article {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    private User user;

    @OneToMany(mappedBy = "article")
    private Set<Comment> comments;

    @OneToMany(mappedBy = "article")
    private Set<Reaction> reactions;

    // Other fields like content, type, etc.
}

@Entity
public class Comment {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "article_id")
    private Article article;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    private User user;

    // Other fields like text, timestamp, etc.
}

@Entity
public class Reaction {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "article_id")
    private Article article;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id")
    private User user;

    // Other fields like type, etc.
}

@Entity
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @OneToMany(mappedBy = "user")
    private Set<Article> articles;

    // Other user fields like name, username, email, etc.
}

This method should return a page of articledetail objects, each containing the article details, author, comments and reactions. However, the problem I'm facing is that the comments and reactions are not aggregated correctly under their respective articles. Each articledetail instance should contain a list of commentdetail and reactiondetail , but they are returned as separate entries.

Is there a way to construct this query to correctly aggregate the comments and reactions under their respective articles? Or should it be processed programmatically after getting the data?

Any suggestions or alternatives would be greatly appreciated!

Solution

The solution I implemented involves a two-step query process using querydsl:

First, I get the ids of articles that meet certain conditions:

var articleids = jpaqueryfactory
        .select(article.id)
        .from(article)
        // conditions and joins
        .fetch();

var articles = jpaQueryFactory
        .select(article)
        .from(article)
        // Joins for comments and reactions
        .where(article.id.in(articleIds))
        .transform(groupBy(article.id).list(
            Projections.constructor(ArticleDetail.class,
                // Projection fields
            )));

return new PageImpl<>(articles, pageRequest, articles.size());

Key aspects of the solution: Utilize a two-step query process to first obtain the article id and then retrieve the corresponding article by aggregating the data. Using groupby().list() as well as projections.constructor() is crucial to correctly aggregating comments and reactions under each post. This approach effectively solves the problem of aggregating comments and reactions under their respective articles while ensuring efficient data acquisition and paging.

I hope this detailed explanation helps others facing similar situations. Feedback or suggestions for further optimization are always welcome!

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