schema.rb

PostcardSchema = Dry::Validation.Schema do
      configure do
        config.messages_file = (ForbiddenWord).root.join('config/locales/errors.yml')
        def state_required?(country)
          country.is_state_required
        end
      end
      required(:address).filled
      required(:city).filled
      required(:zip_code).filled(format?: ZIP_CODE_FORMAT)
      required(:content).filled(min_size?: MINIMAL_CONTENT_LENGTH)
      required(:country).filled
      required(:country).maybe(:state_required?)

      rule(country_requires_state: [:country, :state]) do |country, state|
        country.state_required?.then(state.filled?)
      end
    end