validates with class methods ( validates_* なヘルパーはクラスメソッドです！)

dummy_model.rb

# -*- mode: ruby -*-

class Klass < ActiveRecord::Base
  # "undefined ..."
  validates_inclusion_of :in => values

  # success !
  validates_inclusion_of :in => self.values

  # cannot be called
  def values; end

  # can be called
  def self.values; end
end