old_contents: 
  /**
   * Module dependencies.
   */
  
  var mongoose = require('mongoose')
    , async = require('async')
    , Article = mongoose.model('Article')
    , User = mongoose.model('User')
  
  /**
   * Clear database
   *
   * @param {Function} done
   * @api public
   */
  
  exports.clearDb = function (done) {
    var callback = function (item, fn) { item.remove(fn) }
  
    async.parallel([
      function (cb) {
        User.find().exec(function (err, users) {
          async.forEach(users, callback, cb)
        })
      },
      function (cb) {
        Article.find().exec(function (err, apps) {
          async.forEach(apps, callback, cb)
        })
      }
    ], done)
  }
  
new_contents: 
  /**
   * Module dependencies.
   */
  
  var mongoose = require('mongoose')
    , async = require('async')
    , Article = mongoose.model('Article')
    , User = mongoose.model('User')
  
  /**
   * Clear database
   *
   * @param {Function} done
   * @api public
   */
  
  exports.clearDb = function (done) {
    async.parallel([
      function (cb) {
        User.collection.remove(cb)
      },
      function (cb) {
        Article.collection.remove(cb)
      }
    ], done)
  }
  
subject: use collection.remove to empty collections