We need a way for ids to not have to be unwrapped from $oid before they are assigned as an ObjectId.

Incoming data:

{"_id":{"$oid":"..."}}

Realm class:

class Item: Object {
@objc dynamic var _id = ObjectId.generate()
}

That's all that should be needed.

So the default ObjectId class needs:

enum CodingKeys: String, CodingKey {
case oid = "$oid"
}

then in init(from decoder:Decoder)

let container = try decoder.container(keyedBy: CodingKeys.self)
try super.init(string: try container.decode(String.self, forKey: .oid))

Do that when the .singleValueContainer() call fails.
That way both use cases are satisfied.