Mongoose NodeJS Schema with array of ref's - node.js

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