validates :something, validator: fallback

validates_is.rb

# Adds a fallback <tt>:is</tt> option to +validates+.
#
#   # validates :preferences, class: Hash
#   class ClassValidator < ActiveModel::EachValidator
#     def validate_each(record, attribute, value)
#       record[attribute].is_a?(options[:is]) ||
#         record.errors.add attribute, options[:message] ||
#           record.errors.generate_message(attribute, :class)
#     end
#   end
ActiveModel::Validations::ClassMethods.module_eval do
  protected

  def _parse_validates_options_with_fallback(options)
    _parse_validates_options_without_fallback(options) || { is: options }
  end

  alias_method_chain :_parse_validates_options, :fallback
end