Algolia_Algoliasearch_Helper_Entity_Categoryhelper::getRootCategoryId PHP Method

    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;
    }