Model.create not setting values on parent polymorphic model

[Blacksmoke16]

module EveToolsApi::Models
  abstract class Contract
    include Clear::Model
    self.table = "contracts"

    column contract_id : Int32, primary: true

    polymorphic EveToolsApi::Models::Courier, EveToolsApi::Models::ItemExchange, through: "type"
  end
end

module EveToolsApi::Models
  class Courier < EveToolsApi::Models::Contract
    column reward : Int32
  end
end

module EveToolsApi::Models
  class ItemExchange < EveToolsApi::Models::Contract
    column price : Int32
  end
end

ItemExchange.create({contract_id: 3, price: 123})

pg      | 2018-09-02 22:53:53.905 UTC [3486] ERROR:  null value in column "contract_id" violates not-null constraint
pg      | 2018-09-02 22:53:53.905 UTC [3486] DETAIL:  Failing row contains (null, EveToolsApi::Models::ItemExchange, 0, 123).
pg      | 2018-09-02 22:53:53.905 UTC [3486] STATEMENT:  INSERT INTO "contracts" ("type", "price") VALUES ('EveToolsApi::Models::ItemExchange', 123) RETURNING *
api2    | Error catched, last request was:
api2    | INSERT INTO "contracts" ("type", "price") VALUES ('EveToolsApi::Models::ItemExchange', 123) RETURNING *