ObjectId dereferencing in JavaScript for MongoDB

gistfile1.js

var _deref = function (doc, field, col) {
    var oid = ObjectId(doc[field]);
    delete doc[field];
    doc[field] = db[col].findOne({_id:oid});
    return doc;
}

var deref = function(field, collection){ 
    // C-C-C-Closure!!
    return function(doc){ 
        return _deref(doc, field, collection); 
    }
} 

For dereferencing ObjectIds in mongoDB