modifying parent's validator in child model

Hello,
I need to remove a validator from a parent model in a child model.
Tried _validators.delete, but that does not seem to work and
validation on parent class is still called.

class Order < ActiveRecord::Base
  validate :has_available_shipment
end

class TradeOrder < Order
  # tried this from somebody's advice
  self._validators.delete(:has_available_shipment)
end