Fetching Multiple To-Many Relationships with jOOQ MULTISET

jOOQ is a popular Java Object-oriented Querying library that allows developers to write typesafe SQL queries. In this article, we will explore how to fetch multiple to-many relationships with jOOQ MULTISET.

The approach taken by jOOQ is truly revolutionary since it offers a solution that allows you to fetch multiple to-many relationships using a single type-safe query.

Domain Model

Let’s assume that we have the following database tables:

CREATE TABLE post (
    id BIGINT PRIMARY KEY,
    title VARCHAR(255) NOT NULL
);

CREATE TABLE tag (
    id BIGINT PRIMARY KEY,
    name VARCHAR(255) NOT NULL
);

CREATE TABLE post_tag (
    post_id BIGINT NOT NULL,
    tag_id BIGINT NOT NULL,
    PRIMARY KEY (post_id, tag_id),
    FOREIGN KEY (post_id) REFERENCES post(id),
    FOREIGN KEY (tag_id) REFERENCES tag(id)
);

We have a many-to-many relationship between post and tag entities, which is represented by the post_tag join table.

Fetching Multiple To-Many Relationships

With jOOQ, we can use the MULTISET function to fetch multiple to-many relationships in a single query. Here’s an example:

List<Post> posts = dsl.selectFrom(POST)
    .leftJoin(POST_TAG).on(POST.ID.eq(POST_TAG.POST_ID))
    .leftJoin(TAG).on(POST_TAG.TAG_ID.eq(TAG.ID))
    .groupBy(POST.ID)
    .orderBy(POST.ID)
    .fetchInto(Post.class, 
        POST.ID, POST.TITLE, 
        multiset(TAG.ID, TAG.NAME).convertFrom(r -> 
            new Tag(r.get(TAG.ID), r.get(TAG.NAME))
        )
    );

In the above query, we’re fetching all posts and their associated tags using the MULTISET function. The multiset function takes two arguments: the columns to fetch and a lambda function to convert the fetched rows into the desired object.

In this case, we’re converting the fetched rows into a Tag object using the r -> new Tag(r.get(TAG.ID), r.get(TAG.NAME)) lambda function.

Conclusion

In this article, we’ve seen how to fetch multiple to-many relationships with jOOQ MULTISET. This approach allows us to avoid Cartesian Products and write type-safe queries that are easy to maintain.

jOOQ is a powerful library that can help developers write complex SQL queries with ease. If you’re interested in learning more about jOOQ, be sure to check out their official documentation.