Duplicate a document in MongoDB using a new _id

Ok, I suppose that this is a silly question and probably has a simple answer.

How can I duplicate a document in MongoDB, changing the _id of the new one?

Imaging that you have the original document:

> var orig = db.MyCollection.findOne({_id: 'hi'})

And now I want another document in the collection with _id 'bye'.

Just change the id and re-insert.

> db.coll.insert({_id: 'hi', val: 1})
> var orig = db.coll.findOne({_id: 'hi'})
> orig._id = 'bye'
bye
> db.coll.insert(orig)
> db.coll.find()
{ "_id" : "hi", "val" : 1 }
{ "_id" : "bye", "val" : 1 }

You can give a new ObjectId to the copy Document. In mongo shell

var copy = db.collection.findOne();
for (var i = 0; i< 30; i++){ 
    copy._id = new ObjectId(); 
    db.collection.insert(copy);
}