public function leaves() {
    $grammar = $this->getConnection()->getQueryGrammar();

    $rgtCol = $grammar->wrap($this->getQualifiedRightColumnName());
    $lftCol = $grammar->wrap($this->getQualifiedLeftColumnName());

    return $this->descendants()
                ->whereRaw($rgtCol . ' - ' . $lftCol . ' = 1');
  }