An error renderer for ActiveModel models errors

errors_mapper.rb

class ErrorsMapper
  class << self
    def call(model, *prefix)
      model.errors.details.map { |attribute, detail| map_details(model, prefix, attribute, detail) }.flatten
    end

    def single(status: 422, attribute: :base, code: :invalid, message: I18n.t('activerecord.errors.messages.invalid'))
      { status: status, pointer: attribute, code: code, detail: message }
    end

    private

    def map_details(model, prefix, attribute, detail)
      detail.map.with_index do |error, index|
        if error[:value].respond_to?(:errors)
          call(error[:value], [*prefix, attribute])
        else
          code = error[:error]
          message = full_message(model, attribute, index, code)
          single(attribute: [*prefix, attribute].compact.join('.'), code: code, message: message)
        end
      end
    end

    def full_message(model, attribute, index, code)
      message = model.errors.full_message(attribute, model.errors.messages[attribute][index])
      message += allowed_values(model, attribute) if code == :inclusion
      message
    end

    def allowed_values(model, attribute)
      validator = model.class.
                  validators_on(attribute).
                  find { |v| v.is_a?(ActiveModel::Validations::InclusionValidator) }
      if validator
        values = validator.options[:in]
        I18n.t('activerecord.errors.messages.inclusion_addition', values: values.join(', '))
      else
        ''
      end
    end
  end
end