var _ = require('lodash');

db.users.find({}).forEach(function(user){
  var newNewMessages = {};

  _.each(user.newMessages, function(val, key){
    if(key != "undefined"){
      newNewMessages[key] = val;
    };
  });

  db.users.update({_id: user._id}, {$set: {'newMessages': newNewMessages}});
});