public function getFieldDescriptor($key)
    {
        if (array_key_exists($key, $this->fieldDescriptors)) {
            return $this->fieldDescriptors[$key];
        } else if (array_key_exists($key, $this->orderFieldDescriptors)) {
            return $this->orderFieldDescriptors[$key];
        } else {
            throw new ShippingException('field descriptor with key ' . $key . ' could not be found');
        }
    }