model.js

var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/test');

var Schema = mongoose.Schema;

var PageSchema = Schema({
  title: { type: String, default: '', trim: true },
  url:   { type: String, default: '', trim: true },
  thumbnails: {type: Schema.ObjectId,  ref: 'Thumbnail'},
});

var Page = mongoose.model('Page', PageSchema);

Page.schema.post('save', function(doc) {
  if(doc.thumbnails) return;
  Thumbnail.generate(doc.url, function(err, thumb){
    if (err)
      throw err; // TODO: deal with error
    doc.thumbnails = thumb;
    doc.save();
  });
});

var ThumbnailSchema = Schema({
  domain: {type: String, trim: true},
  url: {
    small: { type: String, trim: true },
    original: {type: String, trim: true }
  }
});

ThumbnailSchema.statics.generate = function(doc, cb) {
  // generate site preview , save the thumbnail and call cb
  cb(null, new Thumbnail());
};

var Thumbnail = mongoose.model('Thumbnail', ThumbnailSchema);

model_test.js

require('../');
var assert = require('assert');
var mongoose = require('mongoose');
var Page = mongoose.model('Page');
var Thumbnail = mongoose.model('Thumbnail');

it('generates thumbnail if page does not have one', function(done) {

  // Add another post save hook to page.
  // Becase it doesn't have a thumbnail, so one should be created, added
  // to the page and the page will be saved again, so we can expect this
  // to fire twice.
  var pageAttr = { title: 'Gist web page', url: 'http://www.gist.github.com' };
  var count = 0, expected = 2;
  var page = new Page(pageAttr);
  page.on('save', postPageSaveA);
  page.save();

  function postPageSaveA(doc) {
    ++count;
    assert(count <= expected, 'post saved fired too many times');

    // delay the next test to allow this test to fail for firing too many times
    if (count === expected)
      setTimeout(doUpdate, 400);
  }

  function doUpdate() {
    // Now change the doc and save it again, now it should fire only once
    page.url += '/changed';
    page.removeListener('save', postPageSaveA);
    count = 0; expected = 1;
    page.on('save', postPageSaveB);
    page.save();
  }

  function postPageSaveB(doc) {
    ++count;
    assert(count <= expected, 'post saved fired too many times');

    // same thing, delay finish to allow failure
    if (count === expected)
      setTimeout(done, 400);
  }
});