MangoDB $lookup 多表查询，

SELECT
    * 
FROM
    COURSE 
WHERE
    Course_number IN ( SELECT Pererquisite_number FROM PREREQUISITE WHERE Course_number = 'CS3308' )

查询以检索“CS3308”课程的先决条件的课程名称。

课程表是主表
PREREQUISITE表是课程先决表。请问这个用MangoDB aggregate 的$lookup怎么查询？

db.orders.aggregate([
   {
     $lookup:
       {
         from: "inventory",
         localField: "item",
         foreignField: "sku",
         as: "inventory_docs"
       }
  }
])

SELECT *, inventory_docs
FROM orders
WHERE inventory_docs IN (SELECT *
FROM inventory
WHERE sku= orders.item);

我看过这个结构，orders.item 我怎么修改为固定的值CS3308？