gistfile1.js

migrate_one_faster = function(c) { 
  print("Collection: " + c);
  print("Initial Count: " + db[c].count());
  i = 0;
  db[c].find({created_at:{$exists:true}}).forEach(function(item){
    if(i++ % 10000 == 0){ print("progress: " + i);}
    item.timestamp = item.created_at;
    delete item.created_at;
    db[c].save(item);
    print(tojson(db.getLastErrorObj()));
  });
}

//prints this:
{
	"theshard" : "set1/mongo-1a.public.aws.igodigital.net:27017,mongo-1b.public.aws.igodigital.net:27017,mongo-1d.public.aws.igodigital.net:27017",
	"err" : null,
	"n" : 0,
	"lastOp" : NumberLong(0),
	"ok" : 1,
	"singleShard" : "set1/mongo-1a.public.aws.igodigital.net:27017,mongo-1b.public.aws.igodigital.net:27017,mongo-1d.public.aws.igodigital.net:27017"
}