Mongoose Populate が動作しない 質問する

こんにちは。このスキーマ (schema.js と呼ばれます) があります。

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

var RoomSchema = new Schema({
  name: { type: String, required: true, index: { unique: true } },
  people: { type: Number, required: true },
  childrens: {type: Number, required: true},
  total: {type: Number, required: true}
});

var Room = mongoose.model('Room', RoomSchema);

var AvSchema = new Schema({
  roomId:  {type: Schema.Types.ObjectId, ref: 'Room'},
  people: { type: Number, required: true },
  childrens: {type: Number, required: true},
  total: {type: Number, required: true}
});

var Av = mongoose.model('Av', AvSchema);

module.exports = {
  Room: Room,
  Av: Av
};

module.exports = function(app) {
  var model = require('../models/Schema');

  app.get('/api/rooms', function(req, res) {
    model.Room.find(function(err, rooms) {
      if (err)
        res.send(err);

      res.json(rooms);
    });
  });


  app.get('/api/av', function(req, res) {
    model.Av.find().populate('roomId').exec(function(err, av) {
      if (err)
        res.send(err);

      res.json(av);
    });
  });
};

[{
  "_id": "5444d0dd9a31437167eea816",
  "name": "Single",
  "people": 1,
  "childrens": 1,
  "total": 4
}, {
  "_id": "5444d1009a31437167eea817",
  "name": "Double",
  "people": 2,
  "childrens": 2,
  "total": 10
}]