Add reference to listings

addrefs.js

var setReferenceId = {
  getNext: function () {
    var seq = db.sequences.findAndModify(  { query: { _id: 'listings' } , update : { $inc : { sequence: 1} },  new: true, upsert: true });
    return seq.sequence;
  },
  getCursor: function () {
    return db.listings.find( { reference_id: { $exists: false } }, {});
  },
  setReference: function (id, ref) {
    return db.listings.update({ _id: id}, { $set: { reference_id: ref }});
  },
  run: function () {
    var self = this;
    var count = 0;
    this.getCursor().forEach( function (doc) { self.setReference(doc._id, self.getNext()); print(++count);  });
  }
}