null_objectid.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');

var dbname = 'testing_objectidNull';
console.log('dbname: %s', dbname);
mongoose.connect('localhost', dbname);
mongoose.connection.on('error', function () {
  console.error('connection error', arguments);
});

var schema = new Schema({
    name: String
  , oid: { type: Schema.Types.ObjectId, default: null }
});

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

mongoose.connection.on('open', function () {
  var a = new A({ name: 'null objectId' });

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

    var b = new mongoose.Types.ObjectId;
    A.findOneAndUpdate({_id : a._id },{oid : b },function(err,doc){
      console.log('updated', doc);
      done(err);
    });
  })
});

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