Aggregate.replaceRoot(object:Object): Aggregate

Support: Small program 2.7.4, cloud function 0.8.1, Web

The aggregation phase. Specify an existing field as the root node for the output, or you can specify a calculated new field as the root node.

Parameters

object: Object

Returns a value

Aggregate

API description

ReplaceRoot is used as follows:

replaceRoot({
    newRoot: <表达式>
})

The expression format is as follows:

Example

Use an existing field as the root node

Let's say we have a schools collection that looks like this:

{
  "_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:

db.collection('schools')
  .aggregate()
  .replaceRoot({
    newRoot: '$teachers'
  })
  .end()

The output is as follows:

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

Use the calculated new field as the root node

Let's say we have a roles collection that looks like this:

{ "_id": 1, "first_name": "四郎", "last_name": "黄" }
{ "_id": 2, "first_name": "邦德", "last_name": "马" }
{ "_id": 3, "first_name": "牧之", "last_name": "张" }

The following code uses replaceRoot to first_name between last_name and the following:

const { concat } = db.command.aggregate
db.collection('roles')
  .aggregate()
  .replaceRoot({
    newRoot: {
      full_name: concat(['$last_name', '$first_name'])
    }
  })
  .end()

The output is as follows:

{ "full_name": "黄四郎" }
{ "full_name": "马邦德" }
{ "full_name": "张牧之" }