MongoDB $count

Summary: in this tutorial, you’ll learn how to use the MongoDB $count to return the number of documents in a group.

Introduction to the MongoDB $count

MongoDB $count returns the number of documents in a group. Here’s the syntax of the $count:

{ $count: {} }
Code language: PHP (php)

Note that the $count does not accept any parameters.

The $count is functionally equivalent to using the following $sum in the $group stage:

{ $sum: 1 }
Code language: PHP (php)

MongoDB $count examples

We’ll use the following sales collection to demonstrate the $count:

db.sales.insertMany([
	{ "_id" : 1, "item" : "Americanos", "price" : 5, "size": "Short", "quantity" : 22, "date" : ISODate("2022-01-15T08:00:00Z") },
	{ "_id" : 2, "item" : "Cappuccino", "price" : 6, "size": "Short","quantity" : 12, "date" : ISODate("2022-01-16T09:00:00Z") },
	{ "_id" : 3, "item" : "Lattes", "price" : 15, "size": "Grande","quantity" : 25, "date" : ISODate("2022-01-16T09:05:00Z") },
	{ "_id" : 4, "item" : "Mochas", "price" : 25,"size": "Tall", "quantity" : 11, "date" : ISODate("2022-02-17T08:00:00Z") },
	{ "_id" : 5, "item" : "Americanos", "price" : 10, "size": "Grande","quantity" : 12, "date" : ISODate("2022-02-18T21:06:00Z") },
	{ "_id" : 6, "item" : "Cappuccino", "price" : 7, "size": "Tall","quantity" : 20, "date" : ISODate("2022-02-20T10:07:00Z") },
	{ "_id" : 7, "item" : "Lattes", "price" : 25,"size": "Tall", "quantity" : 30, "date" : ISODate("2022-02-21T10:08:00Z") },
	{ "_id" : 8, "item" : "Americanos", "price" : 10, "size": "Grande","quantity" : 21, "date" : ISODate("2022-02-22T14:09:00Z") },
	{ "_id" : 9, "item" : "Cappuccino", "price" : 10, "size": "Grande","quantity" : 17, "date" : ISODate("2022-02-23T14:09:00Z") },
	{ "_id" : 10, "item" : "Americanos", "price" : 8, "size": "Tall","quantity" : 15, "date" : ISODate("2022-02-25T14:09:00Z")}
]);
Code language: JavaScript (javascript)

1) Using MongoDB $count to count the number of documents per group example

The following example uses the MongoDB $count to return the number of items in the sales collection:

db.sales.aggregate([
  {
    $group: {
      _id: '$item',
      itemCount: { $count: {} },
    },
  },
])
Code language: PHP (php)

Output:

[
  { _id: 'Mochas', itemCount: 1 },
  { _id: 'Americanos', itemCount: 4 },
  { _id: 'Lattes', itemCount: 2 },
  { _id: 'Cappuccino', itemCount: 3 }
]
Code language: JavaScript (javascript)

In this example:

• _id: "$item" groups the documents by the item field value. It returns four groups for Mochas, Americanos, Lattes, and Cappuchino.
• $count: {} assigns the number of documents that have the same item field value to the itemCount field.

2) Using MongoDB $count with the $match example

The following example uses the $count to calculate the number of documents per item and returns the item with a count greater than two:

db.sales.aggregate([
  {
    $group: {
      _id: '$item',
      itemCount: { $count: {} },
    },
  },
  {
    $match: { itemCount: { $gt: 2 } },
  },
]);
Code language: PHP (php)

Output:

[
  { _id: 'Americanos', itemCount: 4 },
  { _id: 'Cappuccino', itemCount: 3 }
]
Code language: JavaScript (javascript)

Summary

• Use MongoDB $count to return the number of documents in a group.