event.rb

class Event < Node
end

gistfile1.txt

class Account < ActiveRecord::Base
  has_many :nodes
end

Account.first.nodes.create! :type => 'event'

message.rb

class Message < Node
end

node.rb

class Node < ActiveRecord::Base
  belongs_to :account

  def new(attributes = {}, *args, &block)
    clazz = class_for(attributes.delete(:type))
    if clazz == self
      super(attributes, *args, &block)
    else
      clazz.new(attributes, *args, &block)
    end
  end

  def class_for(t)
    case t
    when 'message' then Message
    when 'event' then Event
    else self
    end
  end
end