test.js

var mongoose = require('mongoose');
var db = mongoose.createConnection('127.0.0.1', 'test');

// Make schema
var schema = mongoose.Schema({ name: String });

// Create subscribers collection
var subscribers = [];

// Hook `save` post method
schema.post('save', function () {
  console.log(this.name);
  for (var i in subscribers) {
    subscribers[i](this);
  }
  return true;
});

// Add static method for schema
schema.static('subscribe', function (subscriber) {
  subscribers.push(subscriber);
});

// Create model from schema
var Cat = db.model('Cat', schema);

// Subscriber data changes
Cat.subscribe(function (data) {
  console.log('receive data:');
  console.log(data);
});

// Create new cat with name kitty.
new Cat({ name: 'kitty' }).save(function (err) {
  console.log(err);
});