Aggregating Child Entities in a Query Result using JPA and QueryDSL

Question

I'm working on a Java application using JPA and QueryDSL, facing a challenge with a one-to-many relationship query. I have three entities: Article, Comment, and Reaction. Each Article (one) can have multiple Comments and Reactions (many). I need to fetch each article along with its aggregated comments and reactions.

Here's my current method:

    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());
} 

Entities:

@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 is supposed to return a page of ArticleDetail objects, each containing details of the article, its author, comments, and reactions. However, I'm facing an issue where the comments and reactions are not correctly aggregated under their respective articles. Each ArticleDetail instance should contain a list of CommentDetail and ReactionDetail, but they are being returned as separate entries.

Is there a way to structure this query to aggregate the comments and reactions under their respective articles correctly? Or should this be handled programmatically after fetching the data?

Any advice or alternative approaches would be greatly appreciated!

Answer 1

The solution I implemented involves a two-step querying process using QueryDSL:

First, I fetched the IDs of the articles that met 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: Utilizing a two-step query process to first fetch article IDs and then retrieve the corresponding articles with aggregated data. The use of groupBy().list() along with Projections.constructor() was crucial for the correct aggregation of comments and reactions under each article. This approach effectively resolved the issue of aggregating comments and reactions under their respective articles while ensuring efficient data fetching and pagination.

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