// lastMod.js

module.exports = exports = function lastModifiedPlugin (schema, options) {

  schema.add({ lastMod: Date })

  schema.pre('save', function (next) {

    this.lastMod = new Date

    next()

  })

  if (options && options.index) {

    schema.path('lastMod').index(options.index)

  }

}

// game-schema.js

var lastMod = require('./lastMod');

var Game = new Schema({ ... });

Game.plugin(lastMod, { index: true });

// player-schema.js

var lastMod = require('./lastMod');

var Player = new Schema({ ... });

Player.plugin(lastMod);