Fixing the attached test case so that we never inadvertently replace an existing document could be done by adding `{$where: 'false'}` to the criteria (forcing an insert) or more simply falling back to an insert operation. In either case, users with perfectly legitimate use cases might see a uniqueness exception from MongoDB.

An example of "perfectly legitimate" is in the test case. The collection happens to contain a document with an `_id` XYZ and some additional fields. The user creates a new document with nothing more than the `_id` XYZ and flushes it. Maybe they have `@NotSaved` and other migration mappings for those original, additional fields. They shouldn't expect the persist operation to clobber the existing document.