mongodb "migration" script

migration.js

// changes the data in db.collection[from] to
// db.collection[to] and deletes db.collection[from]
// how can I force the execution of the variables from and to?
function changeFieldKey(collection) {
    var docs = db[collection].find({"from-key": {$exists: true}});
    for (var i = 0; i < docs.length(); i++) {
        print(docs[i])
        docs[i]["to-key"] = docs[i]["from-key"];
        delete docs[i]["from-key"];
        print(docs[i])
        db[collection].save(docs[i]);
    }
}

changeFieldKey("apps")