gistfile1.js

var mongoose = require('mongoose')
var assert = require('assert')
mongoose.connect('mongodb://localhost/mongotest')

var VideoSchema = new mongoose.Schema({
    title: String
  , img: String
  , timestamp: { type: Date, default: Date.now }
  , viewCount: Number
  , commentCount: Number
  , voteCount: Number
}, {
  strict: false
})

mongoose.model('Video', VideoSchema)

var Video = mongoose.model('Video')

var a = new Video({ title: 'Test 1', commentCount: 1})
  , b = new Video({ title: 'Test 2', commentCount: 2})
  , c = new Video({ title: 'Test 3', commentCount: 3})

a.save(function(err) {
  if (err) throw err
  b.save(function(err) {
    if (err) throw err
    c.save(function(err) {
      if (err) throw err

      Video.find({}).sort({ commentCount: -1 }).limit(3).exec(function(err, records) {
        if (err) throw err
        assert.equal(records[0].commentCount, 3)
        console.log('done')
        process.exit()
      })
    })
  })
})