Writing a subquery for a relation's relation in Objection.js - objection.js

Question

I'm using version 2.2.15 at the moment and I'm following the relation subqueries
recipe. I've outlined my models and their relationships towards the bottom.
Currently, I'm fetching an event as well as it's post count using this query:
const events = await Event.query().select([
'events.*',
Event.relatedQuery('posts')
.count()
.as('numberOfPosts'),
]);
Which works fine - but how can I include the count of each of the event's post's users, that is, the total amount of user's going to an event. I've tried using a refs so far without luck.
Models and their relationships:
Event:
posts: {
relation: Model.HasManyRelation,
modelClass: Post,
join: {
from: 'events.id',
to: 'posts.eventId',
},
},
Post:
event: {
relation: Model.BelongsToOneRelation,
modelClass: Event,
join: {
from: 'posts.eventId',
to: 'events.id',
},
},
users: {
relation: Model.ManyToManyRelation,
modelClass: User,
join: {
from: 'posts.id',
through: {
// users_posts is the join table.
from: 'users_posts.postId',
to: 'users_posts.userId',
},
to: 'users.id',
},
},
User:
posts: {
relation: Model.ManyToManyRelation,
modelClass: Post,
join: {
from: 'users.id',
through: {
// users_posts is the join table.
from: 'users_posts.userId',
to: 'users_posts.postId',
},
to: 'posts.id',
},
},

Answer 1

You should be able to do it like so, i haven't tried it in code so you might need to tweek a few things, what i'd do usually if i have a count that needs to be tracked is keep a field on the posts that increments users in the application logic and value is retrieved much faster, in your case you can just count the users_posts where the postId match and get your users count and save it in that field
const events = await Event.query().select([
'events.*',
Event.relatedQuery('posts')
.count()
.as('numberOfPosts'),
Event.relatedQuery('posts')
.join('users_posts', 'posts.id', '=', 'users_posts.postId')
.join('users', 'users.id', '=', 'users_posts.userId')
.countDistinct('users.id')
.as('numberOfPostUsers'),
]);