db.command.aggregate.last

1. Operator Description

Function: Returns the value of the specified field from the last document in a set of documents. This operation is meaningful only when the set of documents is sorted by some definition (sort).

Declaration: db.command.aggregate.last(<expression>)

Notes:

last can only be used in the group stage and is meaningful only when used with sort.

2. Operator Parameters

Field	Type	Required	Description
-	Expression	Required	An expression is a string in the form of $ + specified field.

3. Sample Code

Suppose the collection students contains the following records:

{ "group": "a", "name": "stu1", "score": 84 }
{ "group": "a", "name": "stu2", "score": 96 }
{ "group": "b", "name": "stu3", "score": 80 }
{ "group": "b", "name": "stu4", "score": 100 }

To get the maximum score value from all records, you can first sort all records by score and then retrieve the last value from the last record.

// Sample code in the Cloud Function environment
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('students')
    .aggregate()
    .sort({
      score: 1
    })
    .group({
      _id: null,
      max: $.last('$score')
    })
    .end()
  console.log(res.data)
}

The returned data result is as follows:

{ "_id": null, "max": 100 }