不能跨越两条车道并线的 doctrine join

假设有四张表，合同contract，订单order，产品明细orderItem, 产品档案product，以显而易见的方式关联在一起：

contract:
  + id
  + customer
  + orders // onetomany to order

order:
  + id
  + contract // manytoone to contract
  + items // onetomany to orderItem

orderItem:
  + id
  + order // manytoone to order
  + product // manytoone to product

product:
  + id
  + name

查询customer和他所购买的产品：

select a.customer, p.name
from App\Entity\Contract a
  join a.order.orderItem.product p

join a.order.orderItem.product p 是不对的，只能join一次，需要修改成：

select a.customer, p.name
from App\Entity\Contract a
  join a.order o
  join o.items it
  join it.product p