1235.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_1235';
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
  , num: Number
});
schema.statics.test = function (id, val, cb) {
  this.findOneAndUpdate({ _id: id }, { $inc: { num: val }}).lean().exec(cb);
}

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

mongoose.connection.on('open', function () {
  var a = new A({ name: '1235' });

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

    A.test(a._id, 10, 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();
  });
}