Plugins
Schemas are pluggable, that is, they allow for applying pre-packaged
capabilities to extend their functionality.
Suppose that we have several collections in our database and want
to add last-modified functionality to each one. With plugins this
is easy. Just create a plugin once and apply it to each Schema:
// 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)
  }
}
// in your schema files
var lastMod = require('./lastMod');
var Game = new Schema({ ... });
Game.plugin(lastMod);
var Player = new Schema({ ... });
Player.plugin(lastMod, { index: true });
In the example above we added last-modified functionality to both the Game and Player schemas. We also took advantage of options passing supported by the plugin() method to dynamically define an index on the Player.
