EntityManager＃merge（）期望参数1是实体对象，给定NULL .

目前我正在完成计算机科学学位，我正在学习英语 .

我有一个问题是从Symfony3中的会话对象中获取名为Waiter的实体 .

$em = $this->getDoctrine()->getManager();
    $session = $this->get('Session'); 
    $waiter = $em->merge($session->get('waiter'));
    return $this->render('default/waitermngr.html.twig',array('admin'=>$waiter));

此代码返回错误EntityManager＃merge（）期望参数1是实体对象，给定NULL .

但是$ session对象不为null，因为 return new Response ((string)print_r($session)); 返回：

Symfony \ Component \ HttpFoundation \ Session \ Session Object（[storage：protected] => Symfony \ Component \ HttpFoundation \ Session \ Storage \ NativeSessionStorage Object（[bags：protected] => Array（[attributes] => Symfony \ Component \ HttpFoundation \ Session \ Attribute \ AttributeBag Object（[name：Symfony \ Component \ HttpFoundation \ Session \ Attribute \ AttributeBag：private] => attributes [storageKey：Symfony \ Component \ HttpFoundation \ Session \ Attribute \ AttributeBag：private] => _sf2_attributes [attributes ：protected] =>数组（[admin] => AppBundle \ Entity \ Waiter对象（[idwaiter：protected] => 21 [loginwaiter：protected] => Admin [passwdwaiter：protected] => 81dc9bdb52d04dc20036dbd8313ed055）））[flashes] = > Symfony \ Component \ HttpFoundation \ Session \ Flash \ FlashBag Object（[name：Symfony \ Component \ HttpFoundation \ Session \ Flash \ FlashBag：private] =>闪烁[闪烁：Symfony \ Component \ HttpFoundation \ Session \ Flash \ FlashBag：private ] => Array（）[storageKey：Symfony \ Component \ HttpFoundation \ Session \ Flash \ FlashBag：privat e] => _sf2_flashes））[started：protected] => 1 [closed：protected] => [saveHandler：protected] => Symfony \ Component \ HttpFoundation \ Session \ Storage \ Proxy \ SessionHandlerProxy Object（[handler：protected] = > Symfony \ Component \ HttpFoundation \ Session \ Storage \ Handler \ NativeFileSessionHandler Object（）[wrapper：protected] => 1 [saveHandlerName：protected] => files）[metadataBag：protected] => Symfony \ Component \ HttpFoundation \ Session \ Storage \ MetadataBag对象（[名称：Symfony \ Component \ HttpFoundation \ Session \ Storage \ MetadataBag：private] => __metadata [storageKey：Symfony \ Component \ HttpFoundation \ Session \ Storage \ MetadataBag：private] => _sf2_meta [meta：protected] = >数组（[u] => 1470214147 [c] => 1470208976 [l] => 0）[lastUsed：Symfony \ Component \ HttpFoundation \ Session \ Storage \ MetadataBag：private] => 1470213715 [updateThreshold：Symfony \ Component \ HttpFoundation \ Session \ Storage \ MetadataBag：private] => 0））[flashName：Symfony \ Component \ HttpFoundation \ Session \ Session：private] =>闪烁[attributeNa me：Symfony \ Component \ HttpFoundation \ Session \ Session：private] => attributes）1`

提前致谢！