MobilePhoneLengthValidator

subclass.rb

class MobilePhoneLengthValidator < ActiveModel::Validations::LengthValidator

  def initialize(*)
    super
    @options = options.dup
  end

  def validate_each(*)
    options.merge!(configured_validations)
    super
  end

  def kind
    :length
  end

  def check_validity!
    # The :maximum key is set dynamically
  end

  private

  def configured_validations
    Country.
      config_for!(:my_settings, :validations, :mobile_phone, :length).
      symbolize_keys
  end
end

wrapper.rb

class MobilePhoneLengthValidator < ActiveModel::EachValidator

  def validate_each(*args)
    validator = ActiveModel::Validations::LengthValidator.new(options)
    validator.validate_each(*args)
  end

  def kind
    :length
  end

  def options
    @options.merge(configuration).merge(attributes: @attributes)
  end

  private

  def configuration
    Country.
      config_for!(:my_settings, :validations, :mobile_phone, :length).
      symbolize_keys
  end
end