`id` as a unique field alongside `_id`

[iebb]

Migrating from Djongo here, and my current data involves - _id: the automatic ObjectId field id: Django-style integer field with a unique index.

As I want to maintain compatibility between Djongo model and MongoEngine, and since MongoEngine relies heavily on the 'id' field, what's the current best practice for MongoEngine to maintain both?