Seems to be a "best practice" of developing with [mongoose](http://mongoosejs.com).

app.js

var mongoose = require('mongoose')

mongoose.connect('mongodb://localhost/mydb', function () {
  console.log('mongodb connected')
})

tweet.js

var mongoose = require('mongoose')
  , Schema = mongoose.Schema
  , User = require('./user')

var TweetSchema = new Schema({
  sender: { type: Schema.Types.ObjectId, ref: User.modelName }
, content: { type: String, required: true }
})

TweetSchema.virtual('created_at').get(function () {
  return this._id.getTimestamp()
})

module.exports = mongoose.model('Tweet', TweetSchema)

user.js

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

var UserSchema = new Schema({
  username: { type: String, required: true, unique: true }
, password: { type: String, required: true }
})

module.exports = mongoose.model('User', UserSchema)