StateMachine for STI-like behavior in ActiveRecord

class Post < ActiveRecord::Base
  validates_presence_of :poster_id, :class_name => "User"

  state_machine :type do
    state :topic do
       validates_presence_of :subject, :forum_id
       has_many :posts, :foreign_key => 'topic_id'
    end
    state :post do
      validates_presence_of :topic_id
      belongs_to :topic, :class_name => "Post"
    end
  end
end