Mongoose NodeJS Schema with array of ref's

Question

I know there is allot's of answers about it but still I didn't quite get the idea. I have CourseSchema :

const CourseSchema = new Schema({
course_name: String,
course_number: {type: String, unique : true },
enrolledStudents:[{
    type: mongoose.Schema.Types.ObjectId,
    ref: 'Student' }]
});

And a StudentSchema :

const StudentSchema = new Schema({
first_name: String,
last_name: String,
enrolledCourses:[{
    type: mongoose.Schema.Types.ObjectId,
    ref: 'CourseSchema'
    }]
});

I want to reffer enrolledStudents at CourseSchema with a student, and enrolledCourses at StudentSchema with a course.

router.post('/addStudentToCourse', function (req, res) {
Course.findById(req.params.courseId, function(err, course){
    course.enrolledStudents.push(Student.findById(req.params.studentId, function(error, student){
        student.enrolledCourses.push(course).save();
    })).save();
});
});

but when posting I get an error:

TypeError: Cannot read property 'enrolledStudents' of null

---

Ok so after readying Query-populate I did that:

router.post('/addStudentToCourse', function (req, res) {

    Course.
    findOne({ _id : req.body.courseId }).
    populate({
        path: 'enrolledStudents'
        , match: { _id : req.body.studentId }
    }).
    exec(function (err, course) {
        if (err) return handleError(err);
        console.log('The course name is %s', course.course_name);
    });
});

And when i'm hitting POST on postman I get on the console:

The course name is intro for cs

but it is loading for ever and later on console I get:

POST /courses/addStudentToCourse - - ms - -

Answer 1

You are missing the populate instruction. For example:

see more about it here

Course.
  findOne({ courseId : req.params.courseId }).
  populate('enrolledStudents').
  exec(function (err, course) {
    if (err) return handleError(err);
    console.log('The course name is %s', course.name);

  });

It is working by using the ref field that "knows" how to populate withput using the push syntax. it is like a foreign key population.

Just call the populate method on the query and an array of documents will be returned in place of the original _ids. you can learn more on the internals of the populate methods in the official docs

Answer 2

that is what i did:

router.post('/addStudentToCourse', function (req, res) {
Student.findById(req.body.studentId, function(err, student){
    if(err) return next(err);
    Course.findById(req.body.courseId, function(err, course){
        if(err) return console.log(err);
        course.enrolledStudents.push(student);
        course.save(function (err) {
            if(err)
                console.log(err);
            student.enrolledCourses.push(course);
            student.save(function (err) {
                if (err)
                    console.log(err);
                else{
                    res.send("worked");
                }
            });
        });

    });
});
});