idea-controller.js

idea: function(req, res, next, id) {
          if (!req.challenge) return next(new Error('Failed to load challenge idea ' + id));
          var challenge = req.challenge;
          req.idea = challenge.ideas.id(id); // DocumentArrays have a special id method for looking up a document by its _id.
          // req.idea = challenge.ideas.id(id).populate('user', 'name username') this is what I want to do
          next();

model.js

'use strict';

/**
 * Module dependencies.
 */
var mongoose = require('mongoose'),
  Schema = mongoose.Schema;

 /**
 * Idea Schema
 *
 * A contest can have multiple ideas
 */
var IdeaSchema = new Schema({
  user: {
    type: Schema.Types.ObjectId,
    ref: 'User'
  },
  challenge: {
    type: Schema.Types.ObjectId,
    ref: 'Challenge'
  },
  created: {
    type: Date,
    default: Date.now
  },
  title: {
    type: String,
    required: true,
    trim: true
  },
  description: {
    type: String,
    required: true,
    trim: true
  },
  permissions: {
    type: Array
  },
  updated: {
    type: Array
  },
  status: String
});

/**
 * Validations
 */
IdeaSchema.path('title').validate(function(title) {
  return !!title;
}, 'Title cannot be blank');

IdeaSchema.path('description').validate(function(description) {
  return !!description;
}, 'description cannot be blank');

/**
 * Statics
 */
IdeaSchema.statics.load = function(id, cb) {
  this.findOne({
    _id: id
  })
  .populate('user', 'name username')
  .populate('challenge', 'title')
  .exec(cb);
};

mongoose.model('Idea', IdeaSchema);


/**
 * Challenge Schema
 */
var ChallengeSchema = new Schema({
  created: {
    type: Date,
    default: Date.now
  },
  title: {
    type: String,
    required: true,
    trim: true
  },
  description: {
    type: String,
    required: true,
    trim: true
  },
  tagName: {
    type: String
  },
  user: {
    type: Schema.Types.ObjectId,
    ref: 'User'
  },
  ideas: [IdeaSchema],
  submissions: {
    type: Number,
    default: 0
  },
  permissions: {
    type: Array
  },
  updated: {
    type: Array
  },
  startDate: {
    type: Date,
    default: Date.now
  },
  endDate: {
    type: Date
  },
  status: String
});

/**
 * Validations
 */
ChallengeSchema.path('title').validate(function(title) {
  return !!title;
}, 'Title cannot be blank');

ChallengeSchema.path('description').validate(function(description) {
  return !!description;
}, 'description cannot be blank');

/**
 * Statics
 */
ChallengeSchema.statics.load = function(id, cb) {
  this.findOne({
    _id: id
  }).populate('user', 'name username').exec(cb);
};

mongoose.model('Challenge', ChallengeSchema);