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$count (aggregation)

Definition

$count

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

Note

Disambiguation

This page describes the $count aggregation pipeline stage. For the $count aggregation accumulator, see $count (aggregation accumulator).

Compatibility

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

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

Syntax

$count has the following syntax:

{ $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 return type is represented by the smallest type that can store the final value of count: integer → long → double

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

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

myCount is the output field that stores the count. You can specify another name for the output field.

If the input dataset is empty, $count doesn't return a result.

Tip

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

Examples

Create a collection named scores with these documents:

db.scores.insertMany( [
   { "_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 this result:

{ "passing_scores" : 4 }

If the input dataset is empty, $count doesn't return a result. The following example doesn't return a result because there are no documents with scores greater than 99:

db.scores.aggregate( [
   { $match: { score: { $gt: 99 } } },
   { $count: "high_scores" }
] )

The C# examples on this page use the sample_mflix database from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started in the MongoDB .NET/C# Driver documentation.

The following Movie class models the documents in the sample_mflix.movies collection:

public class Movie
{
    public ObjectId Id { get; set; }
    public int Runtime { get; set; }
    
    public string Title { get; set; }
    public string Rated { get; set; }
    public List<string> Genres { get; set; }
    public string Plot { get; set; }
    
    public ImdbData Imdb { get; set; }
    public int Year { get; set; }
    public int Index { get; set; }
    
    public string[] Comments { get; set; }
   
    [BsonElement("lastupdated")]
    public DateTime LastUpdated { get; set; }
}

Note

ConventionPack for Pascal Case

The properties in the preceding class are named in Pascal case, but the field names in the MongoDB collection use camel case. To account for this difference, you can use the following code to register a ConventionPack when your application starts:

var camelCaseConvention = new ConventionPack { new CamelCaseElementNameConvention() };
ConventionRegistry.Register("CamelCase", camelCaseConvention, type => true);

To use the MongoDB .NET/C# driver to add a $count stage to an aggregation pipeline, call the Count() method on a PipelineDefinition object.

The following example creates a pipeline stage that counts the number of input documents and returns a document with the count as its value:

var pipeline = new EmptyPipelineDefinition<Movie>()
    .Count();

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