{ "_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：

db.collection('test')
  .aggregate()
  .project({
    isBOnly: $.setDifference(['$B', '$A'])
  })
  .end()

{ "_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] }

测试结果和官方给出的结果不一样啊

https://developers.weixin.qq.com/miniprogram/dev/wxcloud/reference-sdk-api/database/command/aggregate/AggregateCommand.setDifference.html