db.tournament.find({}, { players: 1 }).forEach(function (tour) {
  for (var i in tour.players) {
    var p = tour.players[i];
    p.rating = p.elo;
    delete p.elo;
    tour.players[i] = p;
  }
  db.tournament.update({ _id: tour._id }, { $set: { players: tour.players } });
});