Auto increment field using Astronomy

auto_inc_save.js

var autoIncSave = function(doc, fieldName) {
  while (true) {
    var Class = doc.constructor;
    var Collection = Class.getCollection();

    var sort = {};
    var fields = {};
    sort[fieldName] = -1;
    fields[fieldName] = 1;

    var max = Collection.findOne({}, {
      sort: sort,
      fields: fields,
      transform: null
    })[fieldName];
    var next = max + 1;

    try {

      doc.set(fieldName, next);
      doc.save();
      break;

    } catch (e) {

      if (e.name === 'MongoError' && e.code === 11000) {
        continue;
      }

    }
  }
};

Items = new Mongo.Collection('items');
Item = Astro.Class({
  name: 'Item',
  collection: Items,
  fields: {
    order: 'number'
  },
  indexes: {
    order: {
      fields: {
        order: 1
      },
      options: {
        unique: true
      }
    }
  }
});

var item = new Item();
autoIncSave(item, 'order');