error_presenter.rb

# gem install i18n
require 'i18n'
require 'delegate'

class ErrorPresenter < SimpleDelegator
  attr_reader :root_key
  private :root_key

  def initialize(root_key, model)
    @root_key = root_key
    super(model)
  end

  def hash
    errors.each_with_object({}) do |(attribute, errors), hash|
      hash[attribute] = errors.map do |error|
        I18n.t("errors.#{error[:code]}", error[:options])
      end
    end
  end

  def messages
    hash.each_pair.flat_map do |attribute, errors|
      errors.map do |error|
        [I18n.t("#{root_key}.attributes.#{attribute}.name"), error].join(' ')
      end
    end
  end

  private

  def errors
    __getobj__
  end
end

I18n.backend.store_translations(:en, {
  user: {
    attributes: {
      username: {
        name: 'Username'
      },
      password: {
        name: 'Password'
      }
    }
  },
  errors: {
    length: 'must be between %{min} and %{max} characters long',
    blank: 'can not be blank'
  }
})

errors = {
  username: [
    {
      code: 'length',
      value: '',
      options: {
        min: 1,
        max: 40
      }
    }
  ],
  password: [
    {
      code: 'blank',
      value: '',
      options: {}
    }
  ]
}

errors = ErrorPresenter.new(:user, errors)

errors.hash
# => {:username=>["must be between 1 and 40 characters long"], :password=>["can not be blank"]}
errors.messages
# => ["Username must be between 1 and 40 characters long", "Password can not be blank"]