active_model_each_error_to_be_formattable.rb

require 'active_model'

class ActiveModel::Errors
  module HiddenEntity
    attr_accessor :_entity
  end

  def generate_message_with_format(attribute, type = :invalid, options = {})
    message = generate_message_without_format attribute, type, options

    if message.is_a?(Hash)
      entity, message = message, message[:text]

      message.extend HiddenEntity
      message._entity = entity
    end

    message
  end

  alias_method_chain :generate_message, :format

  def full_message_with_format(attribute, message)
    if message.is_a?(HiddenEntity)
      return message._entity[:text] if attribute == :base

      attr_name = attribute.to_s.tr('.', '_').humanize
      attr_name = @base.class.human_attribute_name(attribute, default: attr_name)

      I18n.interpolate message._entity[:format], attribute: attr_name, message: message._entity[:text]
    else
      full_message_without_format attribute, message
    end
  end

  alias_method_chain :full_message, :format
end

# ja:
#   activerecord:
#     errors:
#       messages:
#         blank:
#           text: "が空ですよ"
#           format: "なにやってるんですか%{attribute}%{message}"
#
# みたいにできるようになる（と思う）