Doctrine 2.0 中未加载的延迟加载属性

I'm using PHP and Doctrine 2.0. All entities work fine, with the exception of the entity-relation detailed below (or other entities are failing where I'm not noticing it).

考虑以下实体:

/** @Entity */
class Target {
  /**
   * @ManyToOne(targetEntity="kentitySource", cascade={"persist"})
   * @JoinColumn(name="basic_vacancy_id", nullable=false)
   * @var kentitySource
   */
  $source;

  ...
}

/** @Entity */
class Source {

  ...
}

现在，当我调用 $target->getSource() 我得到一个 kentityproxykentitySourceProxy 的实例(这是正确的代理类).但是，Source 属性的所有 getter 都返回 NULL.