Calling 'populate' leaves array of ObjectIds instead of array of objects

[Josh]

I'm having a hard time trying to populate an array of sub-documents with MongooseJS. Could someone help?

Here is user.js:

    var mongoose = require("mongoose");

    var userSchema = new mongoose.Schema({
        name: {
            type: String
        },
        positions: [{
            type: mongoose.Schema.ObjectId,
            ref: 'position'
        }]
    });

    userSchema.statics.getUserByID = function (id, callback){
        return this.findOne({_id: id}).populate('positions').exec(callback);
    };

    var user = mongoose.model('user', userSchema);

    module.exports = user;

And here is position.js

    var mongoose = require("mongoose");

    var positionSchema = new mongoose.Schema({
        modifiedDate: {
            type: Date
        }
    });

    var position = mongoose.model('position', positionSchema);

    module.exports = position;

The positions array of the user model isn't being populated when getUserByID is called. Am I missing something obvious here?

Here is what is currently saved in the users collection:

    {
        "__v": 1,
        "_id": {
            "$oid": "531f8ab89938130200433ef8"
        },
        "modifiedDate": {
            "$date": "2014-03-11T22:14:43.174Z"
        },
        "name": "Josh",
        "positions": [
            {
                "$oid": "531f8ad39938130200433efa"
            }
        ]
    }

And here is what's in the positions collection:

    {
        "modifiedDate": {
            "$date": "2014-03-11T22:14:43.163Z"
        },
        "_id": {
            "$oid": "531f8ad39938130200433efa"
        },
        "__v": 0
    }

For reference I'm using "mongoose": "^3.8.8" in my package.json file.