Example of limiting the number of populated documents with where() and elemMatch

gistfile1.js

var requirejs = require('requirejs')
, mongoose = require('mongoose');

requirejs.config({
  baseUrl: __dirname
, nodeRequire: require
});

requirejs([], function() {

  mongoose.connect('localhost/mydb');

  var fixtures = null;
  describe('Returning Comments', function() {

    before(function(done) {
      fixtures = require('pow-mongodb-fixtures').connect('mydb');
      fixtures.clearAllAndLoad(__dirname + '/fixtures', function() {
        done();
      });
    });

    describe('posts and comments', function() {

      it('get identified comments of post only', function(done) {
        var Schema = mongoose.Schema;
        var CommentSchema = new Schema({ content: [String] });
        var PostSchema = new Schema({ comments: [{ type: Schema.Types.ObjectId, ref: 'Comment' }] });

        var Comments = mongoose.model('Comment', CommentSchema);
        var Posts = mongoose.model('Post', PostSchema);

        var comment1 = new Comments({ content: 'eerste regel' });
        comment1.save(function(err) {
          var comment2 = new Comments({ content: 'tweede regel' });
          comment2.save(function(err) {
            var post = new Posts();
            var commentIds = [comment1._id, comment2._id];
            console.log('ids: '+commentIds)
            post.comments.push(comment1);
            post.comments.push(comment2);
            post.save(function(err) {
              var postId = post._id;
              Posts.findById(postId)
              .populate('comments')
              .where('comments', { $elemMatch: {$in: commentIds }})
              .exec(function(err, doc) {
                console.log(doc.comments);
                done();
              });
            });
          })
        });
      });

      after(function() {
        fixtures.clear(function() {});
      });
    });
  });

});