From the model:

field :user_id, type: String, versioned: false
key :user_id

In the DB:

[
{
"_id": "248193",
"user_id": "248193",
"version": 4,
"versions": [

{ "_id": "4e7f52804ce45b1a4a000002", "version": 1 }

,

{ "_id": "4e7f57404ce45b1edc000002", "version": 2 }

,

{ "_id": "4e7f57404ce45b1edc000004", "version": 3 }

]
}
]

a) Aren't revisions expected to store previous ID itself?

b) How can _id field be excluded from versioning?