Postgres reference with non-"id" primary keys

defmodule Moneybadger.Ledger.Expense do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  alias Moneybadger.Ledger.Currency

  attributes do
    uuid_primary_key :id

    attribute :name, :string do
      allow_nil? false
    end

    attribute :amount, :decimal do
      allow_nil? false
    end

    attribute :date, :date

    timestamps()
  end

  relationships do
    belongs_to :currency, Currency do
      allow_nil? false
      attribute_type :string
      destination_attribute :code
    end
  end

  actions do
    defaults [:create, :read]
  end

  code_interface do
    ...
  end

  postgres do
    table "expenses"
    repo Moneybadger.Repo

    references do
      reference :currency, on_delete: :delete, on_update: :update
    end
  end
end

defmodule Moneybadger.Ledger.Currency do
  use Ash.Resource,
    data_layer: AshPostgres.DataLayer

  alias Moneybadger.Ledger.Expense

  attributes do
    attribute :code, :string do
      primary_key? true
      allow_nil? false
    end
  end

  relationships do
    has_many :expenses, Expense
  end

  actions do
    defaults [:create]
  end

  code_interface do
    ...
  end

  postgres do
    table "currencies"
    repo Moneybadger.Repo
  end
end

== Compilation error in file lib/moneybadger/ledger/resources/currency.ex ==
** (ArgumentError) schema does not have the field :id used by association :expenses, please set the :references option accordingly
    (ecto 3.10.3) lib/ecto/association.ex:696: Ecto.Association.Has.struct/3
    (ecto 3.10.3) lib/ecto/schema.ex:1891: Ecto.Schema.association/5
    (ecto 3.10.3) lib/ecto/schema.ex:2032: Ecto.Schema.__has_many__/4