Cannot set the _id field explicitly

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This article on storage optimization talks about using the _id field explicitly to reduce storage space.

I have the following model:

class Game
  include Candy::Piece
end

When I try to do:

Game.new(_id: 1, name: 'Pong')

I get the error:

Mongo::OperationFailure: Mod on _id not allowed

Am I doing something wrong, or is this not supported by Candy? Is there a workaround?