Aggregate.replaceRoot

1. Interface Description

Function: Aggregation stage. Specifies an existing field as the root node of the output, or specifies a newly computed field as the root node.

Syntax: replaceRoot({ newRoot: <expression> })

2. Input Parameters

Parameter	Type	Required	Description
newRoot	Aggregate or Object	Yes	The two formats are detailed below

The expression format is as follows:

Format	Description
<field name>	Specify an existing field as the root node of the output (an error will be reported if the field does not exist)
<object>	Compute a new field and use this new field as the root node

3. Response

Parameter	Type	Required	Description
-	Aggregate	Yes	Aggregation object

4. Sample Code

Using an Existing Field as the Root Node

Suppose we have a schools collection with the following content:

{
  "_id": 1,
  "name": "SFLS",
  "teachers": {
    "chinese": 22,
    "math": 18,
    "english": 21,
    "other": 123
  }
}

The following code uses replaceRoot to output the teachers field as the root node:

const tcb = require("@cloudbase/node-sdk");
const app = tcb.init({
  env: "xxx",
});

const db = app.database();
const { gt, sum } = db.command.aggregate;

exports.main = async (event, context) => {
  const res = await db
    .collection("schools")
    .aggregate()
    .replaceRoot({
      newRoot: "$teachers",
    })
    .end();
  console.log(res.data);
};

The output is as follows:

{
  "chinese": 22,
  "math": 18,
  "english": 21,
  "other": 123
}

Using a Computed New Field as the Root Node

Suppose we have a roles collection with the following content:

{ "_id": 1, "first_name": "Silang", "last_name": "Huang" }
{ "_id": 2, "first_name": "Bond", "last_name": "Ma" }
{ "_id": 3, "first_name": "Muzhi", "last_name": "Zhang" }

The following code uses replaceRoot to concatenate first_name and last_name:

const tcb = require("@cloudbase/node-sdk");
const app = tcb.init({
  env: "xxx",
});

const db = app.database();
const { gt, sum, concat } = db.command.aggregate;

exports.main = async (event, context) => {
  const res = await db
    .collection("roles")
    .aggregate()
    .replaceRoot({
      newRoot: {
        full_name: concat(["$last_name", "$first_name"]),
      },
    })
    .end();
  console.log(res.data);
};

The output is as follows:

{ "full_name": "Huang Silang" }
{ "full_name": "Ma Bangde" }
{ "full_name": "Zhang Muzhi" }