public function targetForeignKey($key = null)
    {
        deprecationWarning(
            'BelongToMany::targetForeignKey() is deprecated. ' .
            'Use setTargetForeignKey()/getTargetForeignKey() instead.'
        );
        if ($key !== null) {
            $this->setTargetForeignKey($key);
        }

        return $this->getTargetForeignKey();
    }