Mongoose document.populate() is not working - node.js

Question

I am using a pretty simple Node/Mongo/Express setup and am trying to populate referenced documents. Consider my schemas for "Courses" which contain "Weeks":
// define the schema for our user model
var courseSchema = mongoose.Schema({
teachers : { type: [String], required: true },
description : { type: String },
previous_course : { type: Schema.Types.ObjectId, ref: 'Course'},
next_course : { type: Schema.Types.ObjectId, ref: 'Course'},
weeks : { type: [Schema.Types.ObjectId], ref: 'Week'},
title : { type: String }
});
// create the model for Course and expose it to our app
module.exports = mongoose.model('Course', courseSchema);
I specifically want to populate my array of weeks (though when I changed the schema to be a single week, populate() still didn't work).
Here is my schema for a Week (which a Course has multiple of):
var weekSchema = mongoose.Schema({
ordinal_number : { type: Number, required: true },
description : { type: String },
course : { type: Schema.Types.ObjectId, ref: 'Course', required: true},
title : { type: String }
});
// create the model for Week and expose it to our app
module.exports = mongoose.model('Week', weekSchema);
Here is my controller where I am trying to populate the array of weeks inside of a course. I have followed this documentation:
// Get a single course
exports.show = function(req, res) {
// look up the course for the given id
Course.findById(req.params.id, function (err, course) {
// error checks
if (err) { return res.status(500).json({ error: err }); }
if (!course) { return res.sendStatus(404); }
// my code works until here, I get a valid course which in my DB has weeks (I can confirm in my DB and I can console.log the referenced _id(s))
// populate the document, return it
course.populate('weeks', function(err, course){
// NOTE when this object is returned, the array of weeks is empty
return res.status(200).json(course);
});
};
};
I find it strange that if I remove the .populate() portion from the code, I get the correct array of _ids back. But when I add the .populate() the returned array is suddenly empty. I am very confused!
I have also tried Model population (from: http://mongoosejs.com/docs/api.html#model_Model.populate) but I get the same results.
Thanks for any advice to get my population to work!

Answer 1

below should return course with populated weeks array
exports.show = function(req, res) {
// look up the course for the given id
Course.findById(req.params.id)
.populate({
path:"weeks",
model:"Week"
})
.exec(function (err, course) {
console.log(course);
});
};
### update: you can populate from instance also ###
Course.findById(req.params.id, function (err, course) {
// error checks
if (err) { return res.status(500).json({ error: err }); }
if (!course) { return res.sendStatus(404); }
// populate the document, return it
Course.populate(course, { path:"weeks", model:"Weeks" }, function(err, course){
console.log(course);
});
});
### Update2: Perhaps even more cleanly, this worked: ###
Course.findById(req.params.id, function (err, course) {
// error checks
if (err) { return res.status(500).json({ error: err }); }
if (!course) { return res.sendStatus(404); }
// populate the document, return it
console.log(course);
}).populate(course, { path:"weeks", model:"Weeks" });

Answer 2

here it seems like you are using course.populate() instead of Course.populate()

Answer 3

Use this code instead of yours,I change only one single word course.populate() to Course.populate()
In your case "course" is instance but you need to use Course(Model)
Course.findById(req.params.id, function (err, course) {
if (err) { return res.status(500).json({ error: err }); }
if (!course) { return res.sendStatus(404); }
// Guys in some case below three-line does not work in that case you must comment these lines and uncomments the last three-line
Course.populate('weeks', function(err, course){
return res.status(200).json(course);
});
// Course.populate({ path:"weeks", model:"Weeks" }, function(err, course){
// return res.status(200).json(course);
// });
};