Join two collection in mongoose using nodejs - node.js

Question

I want to join two collection in MongoDB. I have two collections 1. student 2. course.
student collection:
course collection
I tried some code but that is not working.
This is my code
student.js
router.get('/student/:id', function(req, res){
Student.find({_id:req.params.id}, function(err, user){
res.send(user);
})
})
This is Schema:
student.js
const mongoose = require('mongoose');
let StudentSchema = mongoose.Schema({
name:{
type: String
},
email:{
type: String
},
phone:{
type: String
},
password:{
type: String
},
course :[{
type: mongoose.Schema.Types.ObjectId,
ref: 'Course'
}]
}, { collection: 'student' });
const Student = module.exports = mongoose.model('Student', StudentSchema);
course.js
const mongoose = require('mongoose');
const Schema = mongoose.Schema;
let CourseSchema = mongoose.Schema({
student_id:{
type: String
},
course_name:{
type: String
},
course_cost:{
type: String
}
}, { collection: 'course' });
const Course = module.exports = mongoose.model('Course', CourseSchema);
Result:

Answer 1

you need to query like this :
findOne : to find single document (return object {} )
find : to find multiple documents (return array [])
Student.findOne({_id:req.params.id}, function(err, student){
if(!err && student){
Courses.find({student_id:req.params.id},function(error,courses){
if(!error && courses){
student.courses=courses;
}else{
student.courses=[];
}
res.send(student);
});
}
});
currently you are getting course :[] , because there is no field found in students collection , as you can see in photo-1
you need to set course :["coures_id1","coures_id2"] while inserting a document in student collection.
then you can use mongoose populate to populate course into students
Student.findOne({_id:req.params.id}).populate('course').exec(function(err,student){
res.send(student);
});
so after than no need to store , student_id field in courses collection , as you are getting from students collection.