public function getRootCategoryId()
 {
     if (-1 === self::$_rootCategoryId) {
         $collection = Mage::getResourceModel('catalog/category_collection');
         $collection->addFieldToFilter('parent_id', 0);
         $collection->getSelect()->limit(1);
         $rootCategory = $collection->getFirstItem();
         self::$_rootCategoryId = $rootCategory->getId();
     }
     return self::$_rootCategoryId;
 }