server.js

const express = require('express');
const mongoose = require('mongoose');
const app = express();

mongoose.connect('mongodb://localhost/test');

var Schema = mongoose.Schema;

var personSchema = Schema({
  _id: Schema.Types.ObjectId,
  name: String,
  age: Number,
  stories: [{ type: Schema.Types.ObjectId, ref: 'Story' }]
});

var storySchema = Schema({
  author: { type: Schema.Types.ObjectId, ref: 'Person' },
  title: String,
  fans: [{ type: Schema.Types.ObjectId, ref: 'Person' }]
});

var Story = mongoose.model('Story', storySchema);
var Person = mongoose.model('Person', personSchema);

var author = new Person({
  _id: new mongoose.Types.ObjectId(),
  name: 'Ian Fleming',
  age: 50
});

author.save(function (err) {
  if (err) return handleError(err);
  
  var story1 = new Story({
    title: 'Casino Royale',
    author: author._id    // assign the _id from the person
  });
  
  story1.save(function (err) {
    if (err) return handleError(err);
    // thats it!
  });
});


app.get('./', function(req, res) {
    Story.
      findOne({ title: 'Casino Royale' }).
      populate('author').
      exec(function (err, story) {
        if (err) return handleError(err);
        console.log('The author is %s', story.author.name);
        // prints "The author is Ian Fleming"
        res.json({name: story.author.name});
      });
});

app.listen(3000, ()=> console.log('server listen at 3000'));