原则2组合键和DQL连接

我试图过滤掉父元素由复合主键组成的子元素，父元素名为subject，子元素名为courses:

主题:

class Subject
{
   /**
     * @var integer
     *
     * @Column(type="integer")
     * @Id
     * @GeneratedValue(strategy="NONE")
     */
    protected $id;
    /**
     * @var integer
     *
     * @Column(type="integer")
     * @Id
     */
    protected $customer_id;
    /**
     * @var ArrayCollection
     *
     * @OneToMany(targetEntity="Course", mappedBy="subject")
     */
    private $courses;
    ...
}

:

class Course
{
    ...
    /**
     * @var Subject
     *
     * @ManyToOne(targetEntity="Subject", inversedBy="courses")
     * @JoinColumns({
     *   @JoinColumn(name="subject_id", referencedColumnName="id"),
     *   @JoinColumn(name="subject_customer_id", referencedColumnName="customer_id")
     * })
     */
    private $subject;
}

我有"subject_id"answers"subject_customer_id"，我的问题是，我不能过滤掉课程没有加入主题，当我写这个:

$this->em->createQuery("SELECT c FROM Course c WHERE c.subject = :subject")
    ->setParameters( array('subject' => array( 'subject_id' => $subject_id, 'subject_customer_id' => $subject_customer_id ) ) )
    ->getResult();

我得到以下错误:

Doctrine'ORM'Query'QueryException[0]:一个单值关联具有复合主键的实体的路径表达式不是支持。显式地命名复合主键的组件

我能得到这个工作的唯一原因是通过内部连接主题，例如:

$this->em->createQuery("SELECT c FROM Course c INNER JOIN c.subject s WITH s.id = :subject AND s.customer_id = :customer")
    ->setParameters( array( 'subject' => $subject_id, 'customer' => $customer_id ) )
    ->getResult();

有没有办法在不连接主题的情况下根据主题主键检索课程?