Mongoose: Aggregation return empty value - node.js

Question

I'm using aggregation to join 3 collections. But the result of the join is an empty array[].
This is my model for all 3 collections along with queries for aggregate.
Result of console.log(timetable) return []. I;m using references to refer a field in Timetable with corresponding collection.
Timetable Models
var TimetableSchema = new mongoose.Schema ({
timeslot: {
required: true,
'type': String,
},
classroom :{
type: mongoose.Schema.Types.ObjectId,
ref: 'Classroom'
},
subject :{
type: mongoose.Schema.Types.ObjectId,
ref: 'Subject'
},
teacher :{
type: mongoose.Schema.Types.ObjectId,
ref: 'User'
},
day :{
type:String,
required: true,
},
year :{
type:String,
required: true,
},
session :{
type:String,
required:true,
}
})
Classroom Models
var ClassroomSchema = new mongoose.Schema ({
classroom_name: {
type:String,
required:true,
unique: true,
},
classroom_blok:{
type:String,
required:true,
},
classroom_floor: {
type:String,
required:true,
},
});
Subject
var SubjectSchema = new mongoose.Schema ({
subject_id: {
required: true,
'type': String,
'default': shortid.generate
},
subject_name: {
type:String,
required:true,
},
subject_darjah:{
type:String,
required:true,
}
});
Agregate query
router.get('/today_absentee/view/:id',function(req,res){
Teacher.findById(req.session.userId).exec(function (error, user){
if (error){
return next(error);
}else
{
Teacher.find({_id:req.params.id }).exec(function(err, teacher){
if(err)
{
return next(err);
}else
{
Timetable.aggregate([
{
// This is doing the same thing as the previous .find()
$match: { teacher:req.params.id}
},
{
$lookup:{
from: "Classroom", // other table name
localField: "classroom", // name of users table field
foreignField: "_id", // name of userinfo table field
as: "classroom" // alias for userinfo table
}
},
{ $unwind:"$classroom" }, // $unwind used for getting data in object or for one record only
{
$lookup:{
from: "Subject", // other table name
localField: "subject", // name of users table field
foreignField: "_id", // name of userinfo table field
as: "subject" // alias for userinfo table
}
},
{ $unwind:"$subject" }, // $unwind used for getting data in object or for one record only
// define some conditions here
{
$match:{
$and:[{teacher:req.params.id}]
}
},
// define which fields are you want to fetch
{
$project:{
subject_name : "$subject.subject_name",
classroom : "$classroom.classroom_name",
}
}
]).exec(function(err, timetable)
{
// The query output is such that `classroom.classroom_name`
// value is unique for each document
if (err) throw err;
console.log(currentYear);
console.log(timetable);
res.render('creator_content/today_absentee_id',{timetable:timetable, user:user, teacher:teacher});
});
}
});
}
});
});
Models for all 3 collection.

Answer 1

Import mongoose at the top of your file and then use mongoose.Types.ObjectId() when matching against any object id fields in an aggregate query.
I have also used the $addFields aggregation pipeline to add the 2 fields (subject_name, classroom), and then output them in the next stage using $project.
const mongoose = require("mongoose");
router.get('/today_absentee/view/:id', function(req, res) {
Teacher.findById(req.session.userId).exec(function(error, user) {
if (error) {
return next(error);
} else {
Teacher.find({
_id: req.params.id
}).exec(function(err, teacher) {
if (err) {
return next(err);
} else {
Timetable.aggregate([
{
// This is doing the same thing as the previous .find()
$match: {
teacher: mongoose.Types.ObjectId(req.params.id)
}
},
{
$lookup: {
from: "Classroom", // other table name
localField: "classroom", // name of users table field
foreignField: "_id", // name of userinfo table field
as: "classroom" // alias for userinfo table
}
},
{
$unwind: "$classroom"
}, // $unwind used for getting data in object or for one record only
{
$lookup: {
from: "Subject", // other table name
localField: "subject", // name of users table field
foreignField: "_id", // name of userinfo table field
as: "subject" // alias for userinfo table
}
},
{
$unwind: "$subject"
}, // $unwind used for getting data in object or for one record only
// define some conditions here
{
$match: {
$and: [{
teacher: mongoose.Types.ObjectId(req.params.id)
}]
}
},
// Add new field names
{
$addFields: {
subject_name: "$subject.subject_name",
classroom: "$classroom.classroom_name",
}
},
// define which fields are you want to fetch
{
$project: {
subject_name: 1,
classroom: 1,
}
}
]).exec(function(err, timetable) {
// The query output is such that `classroom.classroom_name`
// value is unique for each document
if (err) throw err;
console.log(currentYear);
console.log(timetable);
res.render('creator_content/today_absentee_id', {
timetable: timetable,
user: user,
teacher: teacher
});
});
}
});
}
});
});