public function findByRoot( $rootParagraphId )
    {
        $root  = ( (int) $rootParagraphId ) ?: null;
        $extra = $this->getMapper()
                      ->findByRoot( $root );

        if ( empty( $extra ) )
        {
            $extra = $this->getMapper()
                          ->create( array(
                              'rootParagraphId' => $root,
                          ) );
        }

        return $extra;
    }