Migrate a mongodb collection to another structure (eg. rename fields)

gistfile1.js

// you can copy the collection from one old db to the new one (if needed)
db.oldcollection.find().forEach(function(d) {db.getSiblingDB('anotherdb')['newcollection'].insert(d);});

// gets all the current docs
var docs = db.oldcollecion.find();

// for each old doc, insert it into the new collection 
docs.forEach(function(old) { var doc = {_id:old._id, ac:old.action}; if(old.advertiserid) doc.adid=old.advertiserid; db.newcollection.insert(doc); });

// removes the old collection
db.oldcollection.drop();