knowIfItemIsAdded.js

var mongoose = require('./../mongoose');

console.error(
    '\n===========', '    mongoose version: ', mongoose.version, '========\n\n'
);

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

var schema = new Schema({
    _users: [Schema.ObjectId]
});

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

mongoose.connection.on('open', function () {
  var id = new mongoose.Types.ObjectId;
  var id2 = new mongoose.Types.ObjectId;

  var a = new A({ _users: [id] });
  a.save(function (err, a) {
    if (err) return console.error(err.stack||err);

    A.update({_id: a._id, _users: {$ne: id2}}, {$addToSet: {_users: id2}}, function (err, numUpdated) {
      if (err) return console.error(err.stack||err);
      console.error('updated %d docs', numUpdated);

      A.findById(a, function (err, doc) {
        if (err) console.error(err.stack||err);
        console.error(doc);

        mongoose.connection.db.dropDatabase(function () {
          mongoose.connection.close();
        });
      });

    })

  })
});