MongoClient stores IDs as string

[DieterDePaepe]

Mongoclient stores the _id field as a string rather than an ObjectId when useObjectId == false (default value).

This behavior was introduced in the pull request for issue #36. See JsonObjectCodec#generateIdIfAbsentFromDocument.

The main issue caused by this is that not all commands (can) ensure this behavior, eg: updateWithOptions, when used as an upsert, will create an ObjectId - see #97 - resulting in multiple id representations in the DB.
An additional disadvantage is that using string values for _id requires more storage space per document than an ObjectId.

[karianna]

@DieterDePaepe We've taken a look and agree that #97 is a bug and have put a help wanted label on that. Hopefully a community contributor fixes it in the holiday break. Please open more issues if you find that useObjectId is not being honoured elsewhere.