function get_identity($id = null)
    {
        $id = (int)$id;
        // cache identities for better performance
        if (!array_key_exists($id, $this->identities)) {
            $result = $this->list_identities($id ? "AND `identity_id` = $id" : '');
            $this->identities[$id] = $result[0];
        }

        return $this->identities[$id];
    }