Moving next state with validation on STI

getmoving.rb

class Order < ActiveRecord::Base
end

class WaitingOrder < Order
  def cancel_order(params)
    params = params.merge(type: CanceledOrder.to_s)
    update_attributes(params)
  end
end

class CanceledOrder < Order
  validates :justification, presence: true
end

class WaitingOrderTest < ActiveSupport::TestCase
  test "should cancel waiting order" do
    p = { justification: nil }
    f = WaitingOrder.new

    assert_not f.cancel_order(p)
    assert_equal 'WaitingOrder', f.type
    assert f.errors[:justification].include?(I18n.t('activerecord.errors.messages.blank'))
  
    p = { justification: 'Too late!' }
    assert f.cancel_order(p)
    assert_equal 'CanceledOrder', f.type
  end
end

class Order < ActiveRecord::Base
  validates :justification, presence: true, if: :is_canceled?

  def is_canceled?
    type == "CanceledOrder"
  end
end

class WaitingOrder < Order
  def cancel(params)
    params = params.merge(type: CanceledOrder.to_s)
    update_attributes(params) if params[:justification].present?
  end
end

class CanceledOrder < Order
end