MongoDB $size

Summary: in this tutorial, you’ll how to use the MongoDB $size operator to match documents that have an array containing a specified number of elements.

Introduction to MongoDB $size operator

The $size is an array query operator that allows you to select documents that have an array containing a specified number of elements.

The $size operator has the following syntax:

{ array_field: {$size: element_count} }
Code language: CSS (css)

In this syntax, you specify the element_count after the $size operator to match all documents where the array_field has exact element_count elements.

MongoDB $size operator examples

We’ll use the following products collection:

db.products.insertMany([
	{ "_id" : 1, "name" : "xPhone", "price" : 799, "releaseDate" : ISODate("2011-05-14T00:00:00Z"), "spec" : { "ram" : 4, "screen" : 6.5, "cpu" : 2.66 }, "color" : [ "white", "black" ], "storage" : [ 64, 128, 256 ] },
	{ "_id" : 2, "name" : "xTablet", "price" : 899, "releaseDate" : ISODate("2011-09-01T00:00:00Z"), "spec" : { "ram" : 16, "screen" : 9.5, "cpu" : 3.66 }, "color" : [ "white", "black", "purple" ], "storage" : [ 128, 256, 512 ] },
	{ "_id" : 3, "name" : "SmartTablet", "price" : 899, "releaseDate" : ISODate("2015-01-14T00:00:00Z"), "spec" : { "ram" : 12, "screen" : 9.7, "cpu" : 3.66 }, "color" : [ "blue" ], "storage" : [ 16, 64, 128 ] },
	{ "_id" : 4, "name" : "SmartPad", "price" : 699, "releaseDate" : ISODate("2020-05-14T00:00:00Z"), "spec" : { "ram" : 8, "screen" : 9.7, "cpu" : 1.66 }, "color" : [ "white", "orange", "gold", "gray" ], "storage" : [ 128, 256, 1024 ] },
	{ "_id" : 5, "name" : "SmartPhone", "price" : 599, "releaseDate" : ISODate("2022-09-14T00:00:00Z"), "spec" : { "ram" : 4, "screen" : 9.7, "cpu" : 1.66 }, "color" : [ "white", "orange", "gold", "gray" ], "storage" : [ 128, 256 ] },
	{ "_id" : 6, "name" : "xWidget", "spec" : { "ram" : 64, "screen" : 9.7, "cpu" : 3.66 }, "color" : [ "black" ], "storage" : [ 1024 ] }
])
Code language: JavaScript (javascript)

1) Using MongoDB $size operator to select documents that have an array containing a number of elements

The following example uses the $size operator to select documents whose color array has two elements:

db.products.find({
    color: {
        $size: 2
    }
}, {
    name: 1,
    color: 1
})
Code language: CSS (css)

It returned the following document:

{ "_id" : 1, "color" : [ "white", "black" ], "name" : "xPhone" }
Code language: JSON / JSON with Comments (json)

2) Using MongoDB $size operator with the $or operator example

The following example shows how to use $size operator with the $or operator to select documents whose the color array has one or two elements:

db.products.find({
    $or: [{
            color: {
                $size: 1
            }
        },
        {
            color: {
                $size: 2
            }
        }
    ]
}, {
    name: 1,
    color: 1
})
Code language: PHP (php)

It returned the following documents:

{ "_id" : 1, "color" : [ "white", "black" ], "name" : "xPhone" }
{ "_id" : 3, "color" : [ "blue" ], "name" : "SmartTablet" }
Code language: JSON / JSON with Comments (json)

Summary

• Use the $size operator to select documents that contains an array with a specified number of elements.