db.command.aggregate.substr

1. Operator Description

Function: Returns a substring of a specified length starting from a specified position in the string. It is an alias for db.command.aggregate.substrBytes, and the latter is recommended.

Declaration: db.command.aggregate.substr([<expression1>, <expression2>, <expression3>])

2. Operator Parameters

Field	Type	Required	Description
-	<Array>Expression	Required	Array of aggregate expressions, with each expression detailed below

Parameter description:

expression1is any valid expression that can be resolved to a string, andexpression2andexpression3` are any valid expressions that can be resolved to numbers.

If expression2 is negative, the result returned is "".

If expression3 is negative, the result returned is a substring starting from the position specified by expression2 and including all subsequent characters.

3. Sample Code

Suppose the collection students contains the following records:

{ "birthday": "1999/12/12", "firstName": "Yuanxin", "group": "a", "lastName": "Dong", "score": 84 }
{ "birthday": "1998/11/11", "firstName": "Weijia", "group": "a", "lastName": "Wang", "score": 96 }
{ "birthday": "1997/10/10", "firstName": "Chengxi", "group": "b", "lastName": "Li", "score": 80 }

Using substr, the year, month, and day information can be extracted from birthday. The code is as follows:

// 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()
    .project({
      _id: 0,
      year: $.substr(['$birthday', 0, 4]),
      month: $.substr(['$birthday', 5, 2]),
      day: $.substr(['$birthday', 8, -1])
    })
    .end()
  console.log(res.data)
}

The returned result is as follows:

{ "day": "12", "month": "12", "year": "1999" }
{ "day": "11", "month": "11", "year": "1998" }
{ "day": "10", "month": "10", "year": "1997" }