云开发数据库lookup查询，只需要books里面的某个字段怎么，就是结果中bookList里面不要_id和title,应该怎么写啊？

const db = cloud.database()
db.collection('orders').aggregate()
  .lookup({
    from: 'books',
    localField: 'book',
    foreignField: 'title',
    as: 'bookList',
  })
  .end()
  .then(res => console.log(res))
  .catch(err => console.error(err))

假设 orders 集合有以下记录：

[
  {"_id":4,"book":"novel 1","price":30,"quantity":2},
  {"_id":5,"book":"science 1","price":20,"quantity":1},
  {"_id":6}
]

books 集合有以下记录：

[
  {"_id":"book1","author":"author 1","category":"novel","stock":10,"time":1564456048486,"title":"novel 1"},
  {"_id":"book3","author":"author 3","category":"science","stock":30,"title":"science 1"},
  {"_id":"book4","author":"author 3","category":"science","stock":40,"title":"science 2"},
  {"_id":"book2","author":"author 2","category":"novel","stock":20,"title":"novel 2"},
  {"_id":"book5","author":"author 4","category":"science","stock":50,"title":null},
  {"_id":"book6","author":"author 5","category":"novel","stock":"60"}
]

结果：

[
  {
    "_id": 4,
    "book": "novel 1",
    "price": 30,
    "quantity": 2,
    "bookList": [
      {
        "_id": "book1",
        "title": "novel 1",
        "author": "author 1",
        "category": "novel",
        "stock": 10
      }
    ]
  },
  {
    "_id": 5,
    "book": "science 1",
    "price": 20,
    "quantity": 1,
    "bookList": [
      {
        "_id": "book3",
        "category": "science",
        "title": "science 1",
        "author": "author 3",
        "stock": 30
      }
    ]
  },
  {
    "_id": 6,
    "bookList": [
      {
        "_id": "book5",
        "category": "science",
        "author": "author 4",
        "stock": 50,
        "title": null
      },
      {
        "_id": "book6",
        "author": "author 5",
        "stock": "60",
        "category": "novel"
      }
    ]
  }
]