How do I find object based on child populated property on mongoose - node.js

Question

I'm new in mongoose.
I have a Schema like this:
const sessionSchema = mongoose.Schema({
createdBy: {
type: mongoose.Schema.Types.ObjectId,
ref: 'User',
},
registers: [
{
type: mongoose.Schema.Types.ObjectId,
ref: 'Register',
},
],
date: {
type: Date,
default: Date.now,
immutable: true,
},
});
And register model is this one:
const registerSchema = mongoose.Schema({
sets: [
{
weight: {
type: Number,
},
weightUnit: {
type: String,
default: 'kg',
},
repetitions: {
type: Number,
},
duration: {
type: Number,
},
},
],
session: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Session',
},
exercise: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Exercise',
},
creationDate: {
type: Date,
default: Date.now,
immutable: true,
},
});
I want to find all the sessions created by a user and has an register with an specificied exercise id. I tried this:
const result = Session.find({createdBy: userId, 'registers.exercise': exerciseId}).populate('registers');
But doesn't work. ¿Any suggestion?
Thanks :P

Answer 1

Try this:
Session.aggregate([
{
$match: { createdBy: userId }
},
{
$lookup: {
from: 'Register',
localField: 'registers',
foreignField: '_id',
as: 'registers'
}
},
{
$match: { 'registers.exercise': exerciseId }
}
])
.then((result) => {
console.log('Result: ', result);
})
.catch((err) => {
console.log('Error: ', err);
});
If you are not familiar with MongoDB aggregation framework, the query above might be a bit foreign to you. However, for cross collection queries like you described in your question, the aggregation framework is your best shot(for now).
The $match is basically doing what you would do with a Model.find(), while the $lookup is doing what you would do with a Model.<query>.populate()
You can read more about MongoDB aggregation framework here.