关于jql子查询的问题，请求大家帮助

下面的查询语句能实现你的需求，另外quality是质量，quantity才是数量

const db = uniCloud.database()  
const dbCmd = db.command  
const $ = dbCmd.aggregate  
const res = await db.collection('book').aggregate().lookup({  
  from: 'order',  
  localField: '_id',  
  foreignField: 'book_id',  
  as: 'order'  
})  
.project({  
  _id: true,  
  name: true,  
  inCount: $.size('$order'),  
  total: $.reduce({  
    input: '$order',  
    initialValue: 0,  
    in: $.sum(['$$value', '$$this.quality'])  
  })  
})  
.end()

或使用jql扩展

const dbJQL = uniCloud.databaseForJQL()  
const book = dbJQL.collection('book').getTemp()  
const order = dbJQL.collection('order').getTemp()  
const res = await dbJQL.collection(book, order)  
  .field('name,size(_id.order) as inCount, reduce(_id.order, 0, sum("$$value","$$this.quality")) as total')  
  .get()