$count (aggregation)

Definition

• $count

New in version 3.4.

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

$count has the following prototype form:

{ $count: <string> }

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

See also

• db.collection.countDocuments()
• $collStats
• db.collection.estimatedDocumentCount()
• count
• db.collection.count()

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.

See also

db.collection.countDocuments() which wraps the$group aggregation stage with a $sum expression.

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:

• The $match stage excludes documents that have ascore value of less than or equal to 80 to pass along thedocuments with score greater than 80 to the nextstage.
• The $count stage returns a count of the remaining documentsin the aggregation pipeline and assigns the value to a field calledpassing_scores.

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

The operation returns the following results:

{ "passing_scores" : 4 }