可捕获的致命错误:类的对象<；类>；无法转换为..中的字符串..vendor doctrineorm

i，我正在创建一个市场，有标签和排序顺序，标签可以正常工作，但当我有排序代码时，它会说：

Catchable Fatal Error: Object of class 'Entity'Category could not beconverted to
string in 'vendor'doctrine'orm'lib'Doctrine'ORM'Query'Expr.php line 568

我在谷歌上搜索，没有人有同样的问题？我的代码：

public function tagAction($tag, $sort) {
    $rep = $this->getDoctrine()
            ->getRepository('RSHubMarketplaceBundle:Modification');
    $tags = $this->getDoctrine()
    ->getRepository('RSHubMarketplaceBundle:Category')
    ->findAll();
    if($tag == -1){
            $stags = $tags;
    } else {
        $tag = $this->getDoctrine()
    ->getRepository('RSHubMarketplaceBundle:Category')
    ->find($tag);
        $stags = array($tag); 
    }
    switch ($sort) {
    case 'popularity':
        $mods = $rep->getByPopularity( array($tag));
        break;
    case 'downloads':
        $mods = $rep->getByDownloads( array($tag));
        break;
    case 'newest':
        $mods = $rep->getByNewest(array($tag));
        break;
    case 'name':
        $mods = $rep->getByName( array($tag));
        break;
    }
    return $this->render('RSHubMarketplaceBundle:Marketplace:index.html.twig',
                    array('categories' => $tags, 'mods' =>$mods,
                            'tag' => $tag));
}

以及我遇到问题的方法：

public function getWithCategoriesOrdered(array $nom_categories, $orderCol, $order) {
    $qb = $this->createQueryBuilder('a');
    $qb->join('a.categories', 'c')
            ->where($qb->expr() // HERE
                    ->in('c.name', $nom_categories));
    $qb->add('orderBy', 'a.'.$orderCol.' '.$order);
    return $qb->getQuery()
            ->getResult();
}