Update a document based on its current values

gistfile1.js

// Type in a mongoshell
// Credits: http://stackoverflow.com/questions/3788256/mongodb-updating-documents-using-data-from-the-same-document/3792958#3792958

db.currencies.find({symbol: "$"}).snapshot().forEach(
    function (e) {
        e.customSymbol = e.code.substr(0, 2) + e.customSymbol;
        db.currencies.save(e);
    }
);