MongoDB $size Operator

MongoDB $size operator finds the total number of elements present in the specified array.

The $size operator in MongoDB is used to find the total number of elements in an array. It is used in aggregation pipeline stages and is important to find the size of the array within documents.

This operator is valuable for various data analysis tasks, allowing users to efficiently count the elements within arrays and perform operations based on the array size

The syntax of the $size Operator in MongoDB is:

{ $size: <expression> }

Here, the given arguments must be a valid expression until it resolves to an array. If the argument of this operator is missing or the argument does not resolve to an array, then this operator will give errors.

Let’s look at some examples of the $size operator in MongoDB to understand the operator better.

In the following examples, we are working with:

Database: w3wiki
Collection: arrayExample
Document: three documents that contain the details in the form of field-value pairs.

Using $size Operator example

In this example, we are going to find the size of the number1 field( whose value is an array) using a $size operator. 

Query:

db.arrayExample.aggregate([
... {$match: {name: "Lolo"}},
... {$project: {number1Size: {$size: "$numbers1"}}}])

Output:

Using $size Operator in the Embedded Document example

In this example, we are going to find the size of the favGame.indoorGames field( whose value is an array) using a $size operator. 

Query:

db.arrayExample.aggregate([
... {$match: {name: "Piku"}},
... {$project: {resultSize: {$size: "$favGame.indoorGames"}}}])

Output:

• The $size operator in MongoDB is used to find the total number of elements present in a specified array.
• If the argument of the $size operator is missing or does not resolve to an array, errors may occur.
• It is an aggregation operator that can be used in the aggregation pipeline stages.
• The $size operator can be used to find the size of arrays in documents, allowing for efficient data analysis and manipulation.