CreateMarketForm.rb

module MarketForms
  Create = Dry::Validation.Form do
    key(:code).required
    key(:name).required
    key(:countries_id).required
    key(:description).required
    key(:organization_id).required
    key(:active).required

    rule(code_organization_uniq: [:code, :organization_id]) do |code, organization_id|
      unique?(code, organization_id)
    end

    configure do
      option :market_repo, WheelsApp.instance["repositories.market"]

      def unique?(organization_id, code)
        market_repo
          .markets
          .by_org(organization_id)
          .where(code: code)
          .one
          .blank?
      end
    end
  end
end

Does this work?

module MarketForms
  Create = Dry::Validation.Form do
    key(:code).required
    key(:name).required
    key(:countries_id).required
    key(:description).required
    key(:organization_id).required
    key(:active).required

    rule(code_organization_uniq: [:code, :organization_id]) do |code, organization_id|
      code.unique_by_organisation_id?(organization_id)
    end

    configure do
      option :market_repo, WheelsApp.instance["repositories.market"]

      def unique_by_organisation_id?(organization_id, code)
        market_repo
          .markets
          .by_org(organization_id)
          .where(code: code)
          .one
          .blank?
      end
    end
  end
end

Works perfectly! Thanks!