title: Mongoose Populate callback undefined
body: <p>I am running a query in Mongo so that I can only display all subcollections of the schema except for the account ID. However, I get undefined as the result of the callback "list_data". This is how my query is written in my routes: </p>
  
  <pre><code>exports.list_data = function(req, res){
    models.Director.find({})
    .populate("full_name").populate("dob").populate("favorite_movies").populate("favorite_camera")
    .exec(function(err, list_data){
      res.render('users', {"dirs": list_data});
    })
  };
  </code></pre>
  
  <p>this is my models:</p>
  
  <pre><code>var mongoose = require('mongoose');
  mongoose.connect('mongodb://localhost/lvsAPI');
  var db = mongoose.connection;
  db.on('error', console.error.bind(console, 'connection error:'));
  
  //set schema
  var Schema = mongoose.Schema;
  
  var directorSchema = new Schema ({
    account_id: Number,
    full_name: String,
    dob: Date,
    favorite_camera: String,
    favorite_movies: String
  });
  
  var Director = mongoose.model("Director", directorSchema);
  
  module.exports = {"Director": Director};
  </code></pre>