Sonata admin bundle获取类对象

Question

我在我的项目管理系统中使用sonata管理包，我试图让类对象获取如下数据：

$this->getSubject()->getName();

然而，出来的是：

FatalErrorException: Error: Call to a member function getName() on a non-object in MyAdmin.php line 169

任何人都知道什么是错的？欣赏！

已更新 21/09/2014 以下是我要做的事情：

public function getExportFields() {
    $this->getSubject()->getPurchasePerItem; //Here return error!
    return [
        $this->getTranslator()->trans('Order Number') => 'id', 
        $this->getTranslator()->trans('First Name') => 'customer.First_name',
        $this->getTranslator()->trans('Last Name') => 'customer.Last_name',
        $this->getTranslator()->trans('Contact') => 'customer.contact',
        $this->getTranslator()->trans('Email') => 'customer.email',
        $this->getTranslator()->trans('Street') => 'customer.street',
        $this->getTranslator()->trans('Town') => 'customer.town',
        $this->getTranslator()->trans('State') => 'customer.state',
        $this->getTranslator()->trans('Postcode') => 'customer.postcode',
        //items may more than one
        $this->getTranslator()->trans('Item') => 'purchasePerItem[0].product.model',
        $this->getTranslator()->trans('Item_Qty') => 'purchasePerItem[0].quantity',
        $this->getTranslator()->trans('Item_price') => 'purchasePerItem[0].product.price',
        $this->getTranslator()->trans('Total') => 'total_amount',
        $this->getTranslator()->trans('Shipping') => 'delivery_fee',
        $this->getTranslator()->trans('Order time') => 'createdAt',
        $this->getTranslator()->trans('Last Update') => 'updatedAt',

        ];