Node.js mysql2 Wrap joined table into array of objects - mysql

Question

I have faced the problem when I have two tables with relation, let's say users and posts, and in my DB I have one user with three posts:
Users table:
id
name
1
John
Posts table:
id
user_id
1
1
2
1
3
1
When i try to retrieve my data with the next SQL query
this.connection.query(SELECT * FROM users U LEFT JOIN posts.user_id P on U.id = P.user.id)
i receive three rows for each post with the user:
{ id: 1, name: 'John', posts.id: 1, user_id: 1 }
{ id: 1, name: 'John', posts.id: 2, user_id: 1 }
{ id: 1, name: 'John', posts.id: 2, user_id: 1 }
I want to map all joined data into an array of objects, just like this:
[
{
id: 1,
name: 'John',
posts: [
{ id: 1, user_id: 1 },
{ id: 2, user_id: 1 },
{ id: 3, user_id: 1 },
]
}
]
I already implemented my own mapper via the javascript reduce method, but I hope this could be done underhood.

Answer 1

During my investigation, I found Mysql aggregate functions documentation. In my case, JSON_ARRAYAGG combined with JSON_OBJECT did the job for me and returned the result:
[
{
id: 1,
name: 'John',
posts: [
{ id: 1, user_id: 1 },
{ id: 2, user_id: 1 },
{ id: 3, user_id: 1 },
]
}
]
Query, I used to retrieve data:
SELECT
user.id,
user.name,
user.age,
JSON_ARRAYAGG(JSON_OBJECT(‘id’, post.id, 'label', post.label)) AS posts
FROM users U
LEFT JOIN posts.user_id P on U.id = P.user.id
GROUP BY U.id
Also, this might be useful for Postgres users:
SELECT
user.id,
user.name,
user.age,
json_agg(json_build_object('id', post.id, 'label', post.label, ..... )) AS posts
FROM ...
JOIN ...
GROUP BY user.id, user.name, user.age;