http://stackoverflow.com/questions/16227003/mongoose-pushall-overwrites-current-array-objects

SO-16227003.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_pushalloverwrite_' + ((Math.random() * 10000000) | 0)
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
  , comments: {
        commentData: [{
            comment: String
          , timeStamp: {type:Date,default:Date.now}
          , commenter: String
        }]
    }
});

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

mongoose.connection.on('open', function () {
  var a = new A({
      name: 'pushalloverwrite'
    , comments: { commentData: [{comment:'first'}]}
  });

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

    A.findById(a, function (err, doc) {
      if (err) return done(err);

      console.log('before', doc.comments);

      doc.update({ $pushAll: {
          'comments.commentData': [{comment: 'changed', commenter: 'you' }]
      }}, {upsert: true}, function (err) {
        if (err) return done(err);

        A.findById(a).lean().exec(function (err, doc) {
          console.log('after', doc.comments);
          done(err);
        })
      })
    });
  })
});

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

output:

dbname: testing_pushalloverwrite_8820340
before { commentData:
   [{ comment: 'first',
     _id: 5183e50bf49df50000000003,
     timeStamp: Fri May 03 2013 09:25:47 GMT-0700 (PDT) }] }
after { commentData:
   [ { comment: 'first',
       _id: 5183e50bf49df50000000003,
       timeStamp: Fri May 03 2013 09:25:47 GMT-0700 (PDT) },
     { comment: 'changed',
       commenter: 'you',
       _id: 5183e50bf49df50000000004,
       timeStamp: Fri May 03 2013 09:25:47 GMT-0700 (PDT) } ] }