oops.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
mongoose.connect('localhost', 'testing_oops');
mongoose.connection.on('error', function () {
  console.error(arguments);
});

var schema = new Schema({
    name: String
});

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

mongoose.connection.on('open', function () {
  var a = new A({ name: 'oops' });
  a.save(function (err, a) {
    if (err) return console.error(err.stack||err);

    /* // option 1
    var b = new A;
    b.init({ _id: a.id });
    b.set({ name: 'new value' });
    b.save(function (err) {
      if (err) throw err;
      A.findById(b, function (err, doc) {
        if (err) throw err;
        console.error(doc);
        mongoose.connection.db.dropDatabase(function () {
          mongoose.connection.close();
        });
      });
    });
    */

    // option 2
    A.update({ _id: a._id }, { $set: { name: 'new val' }}, function (err) {
       A.findById(a, function (err, doc) {
        if (err) throw err;
        console.error(doc);
        mongoose.connection.db.dropDatabase(function () {
          mongoose.connection.close();
        });
      });
    });
  })
});