You should play with your ``mongo`` console a bit and see what it does in this situation. 

If you grab an existing document, then remove the ``_id`` from it and save it again, the server will give it another ``_id`` with a new value and save it back to the collection as a new document. Obviously, sans the ``_id`` this document will actually be a copy of the original.

I think you have two choices here:
* Don't allow them to delete the ``_id`` from this interface.
* Follow the same behavior you see in the ``mongo`` console.

I'd lean towards doing the later, but I'd also encourage discussion between you guys on the merits of both approaches. Somewhat related, keep in mind that ``_id`` isn't always an ``ObjectId`` and can be a custom identifier as well. You need to make sure you're accounting for that to (especially if you're doing a ``to_s`` conversions)