How to mass assign attribute 'type' for STI with validation

active_record_mass_assign_attr_for_sti.rb

class Order < ActiveRecord::Base

  validates_inclusion_of :type, :in => %w{SpecialOrder}

  def self.new(params = {})
    _type = params.delete(:type)
    _new = super(params)
    _new.type = _type.to_s.camelize if _type
    return _new
  end

end

class SpecialOrder < Order

end

order = Order.new :type => "special_order", :other_attribute => ....