public function findAllRootCategories()
    {

        // try to load the categories
        $this->rootCategoriesStmt->execute();

        // initialize the array with the store code as key
        $rootCategories = array();

        // load the available root categories
        $availableRootCategories = $this->rootCategoriesStmt->fetchAll(\PDO::FETCH_ASSOC);

        // prepare the array with the root categories
        foreach ($availableRootCategories as $category) {
            $rootCategories[$category[MemberNames::CODE]] = $category;
        }

        // append the root categories to the cache
        return $rootCategories;
    }