如果有人遇到类似的问题，解决方案：

In case someone runs into a similar issue, here is the solution:

/**
 * @var statistic
 *
 * @ORM\OneToOne(targetEntity="statistic", cascade="ALL")
 * @ORM\JoinColumns({
 *   @ORM\JoinColumn(name="fkStatistic", referencedColumnName="pkId", onDelete="SET NULL")
 * })
 */

onDelete 选项将首先删除关系，然后doctrine将执行级联操作。

The onDelete option will remove the relation first and then doctrine will do the cascade operation.