MongoDB Change Array Type from String to ObjectId

1var cursor = db.suppliers.find(); 
2while (cursor.hasNext()) { 
3  var doc = cursor.next(); 
4  var patchIds = doc.patches.map(function (id) {return ObjectId(id)})
5  db.suppliers.update({_id : doc._id}, {$set : {patches : patchIds }});
6}