/**
  * Perform multiple calls to subresources of specified resource
  *
  * @param string $resourceInstanceId
  * @return Mage_Api2_Model_Response
  */
 protected function _multicall($resourceInstanceId)
 {
     if (!$this->_multicall) {
         $this->_multicall = Mage::getModel('api2/multicall');
     }
     $resourceName = $this->getResourceType();
     return $this->_multicall->call($resourceInstanceId, $resourceName, $this->getRequest());
 }