gistfile2.rb

>> HistoryItem.new(:historytype => "Foo").valid?
Foo
[]
=> true
>> HistoryItem.new(:historytype => "Foo").valid?
Blub
["Foo"]
=> true

history_item.rb

class HistoryItem < ActiveRecord::Base
  cattr_accessor :types

  @@types = []

  validates_inclusion_of :historytype, :in => types

  def after_initialize
    puts "Called"
    puts self.historytype
    p @@types
    @@types << self.historytype
  end
end