# File lib/dm-validations/validators/uniqueness_validator.rb, line 18
      def call(target)
        value = target.send(field_name)

        return true if @options[:allow_nil] && value.blank?

        opts = {
          :fields    => target.model.key,
          field_name => value,
        }

        Array(@options[:scope]).each { |subject| opts[subject] = target.send(subject) }

        resource = DataMapper.repository(target.repository.name) { target.model.first(opts) }

        return true if resource.nil?
        return true if target.saved? && resource.key == target.key

        error_message = @options[:message] || ValidationErrors.default_error_message(:taken, field_name)
        add_error(target, error_message, field_name)

        false
      end