db.command.aggregate.setDifference

1. 操作符描述

功能：输入两个集合，输出只存在于第一个集合中的元素。

声明：setDifference([expression1, expression2])

2. 操作符参数

字段	类型	必填	说明
-	<Array>Expression	是	聚合表达式数组

3. 示例代码

假设集合 test 存在以下数据：

{ "_id": 1, "A": [ 1, 2 ], "B": [ 1, 2 ] }
{ "_id": 2, "A": [ 1, 2 ], "B": [ 2, 1, 2 ] }
{ "_id": 3, "A": [ 1, 2 ], "B": [ 1, 2, 3 ] }
{ "_id": 4, "A": [ 1, 2 ], "B": [ 3, 1 ] }
{ "_id": 5, "A": [ 1, 2 ], "B": [ ] }
{ "_id": 6, "A": [ 1, 2 ], "B": [ {}, [] ] }
{ "_id": 7, "A": [ ], "B": [ ] }
{ "_id": 8, "A": [ ], "B": [ 1 ] }

下面的代码使用 setDifference，找到只存在于 B 中的数字：

// 云函数环境下示例代码
const tcb = require('@cloudbase/node-sdk')
const app = tcb.init({
  env: 'xxx'
})

const db = app.database()
const $ = db.command.aggregate
const _ = db.command

exports.main = async (event, context) => {
  const res = await db
    .collection('test')
    .aggregate()
    .project({
      isBOnly: $.setDifference(['$B', '$A'])
    })
    .end()
  console.log(res.data)
}

{ "_id": 1, "isBOnly": [] }
{ "_id": 2, "isBOnly": [3] }
{ "_id": 3, "isBOnly": [3] }
{ "_id": 4, "isBOnly": [5] }
{ "_id": 5, "isBOnly": [] }
{ "_id": 6, "isBOnly": [{}, []] }
{ "_id": 7, "isBOnly": [] }
{ "_id": 8, "isBOnly": [1] }