public function findPublishedPageBySlug($pageSlug)
    {
        $this->makeSelect();

        if (is_string($pageSlug)) {
            $this->sql .= ' and collection_id is null and slug = ?';
            $this->bindValues[] = $pageSlug;
        } else {
            throw new Exception('Unknown page identifier type');
        }

        $this->sql .= " and published_date <= '{$this->today()}'";

        return $this->findRow();
    }