1088.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var assert = require('assert')

console.log('\n===========');
console.log('    mongoose version: %s', mongoose.version);
console.log('========\n\n');

mongoose.connect('localhost', 'testing_1088');
mongoose.connection.on('error', function () {
  console.error('connection error', arguments);
});

var schema = new Schema({
    title: String
  , slug: String
  , _id: String
});

// custom setter
schema.path('title').set(function (v) {
  this.slug = slugify(v);
  this._id = idify(this.slug);
  return v;
});

function slugify (v) {
  return (v || '').replace(/\s+/g, '')
}

function idify (v) {
  return 'mycustomid_' + v
}

var A = mongoose.model('A', schema);

mongoose.connection.on('open', function () {
  var a = new A({ title: 'try 1' });
  console.log(a); // { title: 'try 1', _id: 'mycustomid_try1', slug: 'try1' }

  a.save(function (err, a) {
    if (err) return done(err);

    A.findById(a._id, function (err, doc) {
      console.error('found', doc);
      done(err);
    });
  })
});

function done (err) {
  if (err) console.error(err.stack);
  mongoose.connection.db.dropDatabase(function () {
    mongoose.connection.close();
  });
}