subpopulate.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var assert = require('assert')

console.log('\n===========');
console.log('    mongoose version: %s', mongoose.version);
console.log('========\n\n');

var dbname = 'testing_subpopdoc';
console.log('dbname: %s', dbname);
mongoose.connect('localhost', dbname);
mongoose.connection.on('error', function () {
  console.error('connection error', arguments);
});

var schema = Schema({ _id: 'string', name: 'string' });
var A = mongoose.model('A', schema);

var BrandSchema = new Schema({
    title: String,
    description: String,
    logo: [{name: String, path: String, src: { type: String, ref: 'A' }}], // <===
    owner: {type: Schema.Types.ObjectId, ref: 'User'}
})
var Brand = mongoose.model('Brand', BrandSchema);

mongoose.connection.on('open', function () {
  A.create({ _id: 'first', name: 'subpopdoc' }, function (err, doc) {
    if (err) return done(err);
    Brand.create({ logo: [{ name:'logo1', path: '.', src: doc._id }]}, function (err, brand) {
      if (err) return done(err);

      console.log('created brand', brand);

      Brand.find().populate('logo.src', 'src').exec(function (err, docs) {
        if (err) return done(err);
        console.log('found', docs[0].logo);
        done();
      });
    });
  })

});

function done (err) {
  if (err) console.error(err.stack);
  mongoose.connection.db.dropDatabase(function () {
    mongoose.connection.close();
  });
}