In MongoDB the count aggregation command counts the number of documents in a collection or a view.

It returns a document that contains the count as well as the command status.

Example

Suppose we have a collection called pets with the following documents:

{ "_id" : 1, "name" : "Wag", "type" : "Dog", "weight" : 20 }
{ "_id" : 2, "name" : "Bark", "type" : "Dog", "weight" : 10 }
{ "_id" : 3, "name" : "Meow", "type" : "Cat", "weight" : 7 }
{ "_id" : 4, "name" : "Scratch", "type" : "Cat", "weight" : 8 }
{ "_id" : 5, "name" : "Bruce", "type" : "Bat", "weight" : 3 }
{ "_id" : 6, "name" : "Fetch", "type" : "Dog", "weight" : 17 }
{ "_id" : 7, "name" : "Jake", "type" : "Dog", "weight" : 30 }

We can use the following query to return the number of documents in the collection:

db.runCommand( { count: "pets" } )

Result:

{ "n" : 7, "ok" : 1 }

The MongoDB documentation actually recommends against using the count command and its wrapper methods without a query predicate. This is because, when run without a query predicate, it returns results based on the collection’s metadata, which may result in an approximate count.

Count the Result of a Query

You can count the result of a query by using the following form.

Example:

db.runCommand( { 
    count: "pets", 
    query: { type: "Dog" } 
} )

Result:

{ "n" : 4, "ok" : 1 }

In this example, we can see that there are four dogs in the collection.

Let’s check the count of another query. This time we’ll find out how many pets have a weight greater than a certain amount.

db.runCommand( { count:'pets',
    query: { weight: { $gt: 10 } }
} )

Result:

{ "n" : 3, "ok" : 1 }

Limit the Count

You can use the limit parameter to specify a maximum number of documents to count.

Example:

db.runCommand( { 
    count: "pets", 
    query: { type: "Dog" },
    limit: 3
} )

Result:

{ "n" : 3, "ok" : 1 }

We saw before that there are actually 4 dogs, but in this example we limited it to count a maximum of 3.

If the limit is higher than the actual count, then the result won’t be affected by the limit argument.

Example:

db.runCommand( { 
    count: "pets", 
    query: { type: "Dog" },
    limit: 10
} )

Result:

{ "n" : 4, "ok" : 1 }

Skipping Documents

You can use the skip parameter to skip a number of documents before counting.

Example:

db.runCommand( { 
    count: "pets", 
    query: { type: "Dog" },
    skip: 2
} )

Result:

{ "n" : 2, "ok" : 1 }

Deprecation

Note that the MongoDB documentation states the following:

MongoDB drivers compatible with the 4.0 features deprecate their respective cursor and collection count() APIs (which runs the count command) in favor of new APIs that corresponds to countDocuments() and estimatedDocumentCount(). For the specific API names for a given driver, see the driver API documentation.

More Information

The count command includes other fields, such as hint, comment, readConcern, and collation.

See the MongoDB documentation for more information.