_id field is not retrieved

[myartsev]

I am trying to retrieve the _id key as part of jsonapi-store-mongodb response so that I can get relationships to work via query population.

All the other mongodb generated fields (_v, createdAt, updatedAt) are successfully returned by jsonapi-store-mongodb if I include them in the schema, however _id appears to be the one outlier.
Is there something special about it?

Tangentially - is there another way to get relationships / reference documents to work?