Aggregate.count(fieldName: string): Aggregate

Support: Small program 2.7.4, cloud function 0.8.1, Web

The aggregation phase. Calculates the number of records entered into this stage by the previous aggregation stage and outputs a record where the value of the specified field is the number of records.

Parameters

fieldName: string

Returns a value

Aggregate

API description

The form of count is as follows:

count(<string>)

The name of the field that outputs the number of records, cannot be an empty string, cannot begin with $, and cannot contain . Character.

The count stage is equivalent to the operation of group-project:

const $ = db.command.aggregate
db.collection('items').aggregate()
  .group({
    _id: null,
    count: $.sum(1),
  })
  .project({
    _id: 0,
  })
  .end()

The above outputs a record that contains the count field.

Example

Suppose the collection items have the following records:

{
  _id: "1",
  price: 10.5
}
{
  _id: "2",
  price: 50.3
}
{
  _id: "3",
  price: 20.8
}
{
  _id: "4",
  price: 80.2
}
{
  _id: "5",
  price: 200.3
}

Find the number of records with a price greater than 50:

const $ = db.command.aggregate
db.collection('items').aggregate()
  .match({
    price: $.gt(50)
  })
  .count('expensiveCount')
  .end()

The results are as follows:

{
  "expensiveCount": 3
}