migrate-with-transform.js

var MongoClient = require('mongodb').MongoClient;

MongoClient.connect('mongodb://localhost/fromdb',function(err,dbLocal) {
	if ( err ) throw err;
	MongoClient.connect('mongodb://remotehost/todb',function(err,dbRemote) {
		if ( err ) throw err;
		var collectionLocal = dbLocal.collection('fromcollection'),
			collectionRemote = dbRemote.collection('tocollection'),
			queryLocal = {field:'value'};
		collectionLocal.find(queryLocal).each(function(err,docLocal) {	
			if ( err ) throw err;
			// transform docLocal
			var queryRemote = {_id:docLocal._id};
			collectionRemote.findAndModify(queryRemote,[['_id',1]],docLocal,{upsert:true,new:true},function(err,docRemote) {
				if ( err ) throw err;
				console.log('upserted %s : %s',docLocal._id,docRemote._id);
			});
		});
	});
});