$count (aggregation)

Definition

$count

New in version 3.4.

Passes a document to the next stage that contains a count of the number of documents input to the stage.

Compatibility

You can use $count for deployments hosted in the following environments:

• MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

• MongoDB Enterprise: The subscription-based, self-managed version of MongoDB

• MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

$count has the following prototype form:

{ $count: <string> }

<string> is the name of the output field which has the count as its value. <string> must be a non-empty string, must not start with $ and must not contain the . character.

Behavior

The $count stage is equivalent to the following $group + $project sequence:

db.collection.aggregate( [
   { $group: { _id: null, myCount: { $sum: 1 } } },
   { $project: { _id: 0 } }
] )

where myCount would be the output field that contains the count. You can specify another name for the output field.

Example

A collection named scores has the following documents:

{ "_id" : 1, "subject" : "History", "score" : 88 }
{ "_id" : 2, "subject" : "History", "score" : 92 }
{ "_id" : 3, "subject" : "History", "score" : 97 }
{ "_id" : 4, "subject" : "History", "score" : 71 }
{ "_id" : 5, "subject" : "History", "score" : 79 }
{ "_id" : 6, "subject" : "History", "score" : 83 }

The following aggregation operation has two stages:

1. The $match stage excludes documents that have a score value of less than or equal to 80 to pass along the documents with score greater than 80 to the next stage.

2. The $count stage returns a count of the remaining documents in the aggregation pipeline and assigns the value to a field called passing_scores.

db.scores.aggregate(
  [
    {
      $match: {
        score: {
          $gt: 80
        }
      }
    },
    {
      $count: "passing_scores"
    }
  ]
)

The operation returns the following results:

{ "passing_scores" : 4 }