Reference subdocument is not getting populated with subdocument which is also in same collection

[sham332]

Below is the schema design code

var mongoose = require('mongoose'),
    Schema = mongoose.Schema;
var ObjectId = mongoose.Schema.Types.ObjectId;

mongoose.set('debug', true);

var chapter = new Schema({
  title:String,
  startPage:Number,
  endPage:Number
});
mongoose.model('chapter', chapter);

var keyPoint = new Schema({
  text:String,
  mentionInChapter:[{type: Schema.ObjectId, ref: 'chapter'}]
});
mongoose.model('keyPoint', keyPoint);

var Book = new Schema({
   title:String,
   author:String,
   chapters:[chapter],
   keyPoints:[keyPoint]
});

mongoose.model('Book', Book);

And when I try to retrieve Book populated with all values, I get blank array. Below is express module containing the query

    exports.getCompleteBook = function(req,res){
        var bookId = '55c65c8749286ac0e25d1f58';
        Book.findById(bookId)
        .lean() 
        .populate({ path: 'keyPoints' }) 
        .exec(function (err, docs) {
            if (err) throw err;
            console.log(docs);
            Book.populate(docs, {path: 'keyPoints.mentionInChapter', model : 'chapter', select:'title'}, function (err, book) {
              if (err) throw err;
              res.json(book);
            });
        });
    };

This link to jist has further on data etc, https://gist.github.com/sham332/20ad1c7b1b460c478700

I should have recieved the 'mentionInChapter' subdocument populated with values, but it is coming blank.

[vkarpov15]

What you're gonna want is:

var Book = new Schema({
   title:String,
   author:String,
   chapters:[{type: Schema.ObjectId, ref: 'chapter'}],
   keyPoints:[{type: Schema.ObjectId, ref: 'keyPoint'}]
});

If you're embedding subdocs, you're not going to be able to run populate() on the array, because the subdocs are stored in the doc itself rather than in a separate collection.

[ogheneovo12]

what if the subdoc contains a reference to an item in another collection,
take for a example a cart with a subdoc of CartItem that references Product in another Collection.

[vkarpov15]

That's fine, as long as Product is a Mongoose model. We have a sample app with a very similar setup to what you're describing, a cart with an array of subdocuments with a productId property that points to a Product model.