How to use MongoDB $mul operator to update documents

Summary: in this tutorial, you’ll learn how to use the MongoDB $mul operator to multiply the value of a field by a number.

Introduction to MongoDB $mul operator

The $mul is a field update operator that allows you to multiply the value of a field by a specified number.

The $mul operator has the following syntax:

{ $mul: { <field1>: <number1>, <field2>: <number2>, ... } }

The <field> that you want to update must contain a numeric value. To specify a field in an embedded document or in an array, you use the dot notation e.g., <embedded_doc>.<field> or <array>.<index> .

If the field doesn’t exist in the document, the $mul operator creates the field and sets its value to zero.

MongoDB $mul operator examples

We’ll use the following products collection:

db.products.insertMany([
    { "_id" : 1, "name" : "xPhone", "price" : 799, "releaseDate": ISODate("2011-05-14"), "spec" : { "ram" : 4, "screen" : 6.5, "cpu" : 2.66 },"color":["white","black"],"storage":[64,128,256]},
    { "_id" : 2, "name" : "xTablet", "price" : 899, "releaseDate": ISODate("2011-09-01") , "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("2015-01-14"), "spec" : { "ram" : 12, "screen" : 9.7, "cpu" : 3.66 },"color":["blue"],"storage":[16,64,128]},
    { "_id" : 4, "name" : "SmartPad", "price" : 699, "releaseDate": ISODate("2020-05-14"),"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("2022-09-14"), "spec" : { "ram" : 4, "screen" : 5.7, "cpu" : 1.66 },"color":["white","orange","gold","gray"],"storage":[128,256]}
 ])

1) Using the MongoDB $mul to multiply the value of a field

The following example uses the $mul operator to multiply the price of the document _id 5 by 10%:

db.products.updateOne({
_id: 5
}, 
{
    $mul: {
        price: 1.1
    }
})

The output document showed that the query matched one document and updated it:

{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

The following query verifies the update:

db.products.find({
    _id: 5
}, {
    name: 1,
    price: 1
})

Output:

{ "_id" : 5, "name" : "SmartPhone", "price" : 658.9000000000001 }

2) Using the MongoDB $mul to multiply the values of array elements

The following query uses the $mul operator to double the values of the first, second, and third array elements in the storage array of the document _id 1:

db.products.updateOne({
    _id: 1
}, {
    $mul: {
        "storage.0": 2,
        "storage.1": 2,
        "storage.2": 2
    }
})

Output:

{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

The following query uses the findOne() method to select the document with _id 1 to verify the update:

db.products.findOne({
    _id: 1
}, {
    name: 1,
    storage: 1
})

Output:

{ "_id" : 1, "name" : "xPhone", "storage" : [ 128, 256, 512 ] }

3) Using the $mul operator to multiply the values of a field in embedded documents

This example uses the $mul operator to double the values of the ram field in the spec embedded documents of all documents from the products collection:

db.products.updateMany({}, {
    $mul: {
        "spec.ram": 2
    }
})

Output:

{ "acknowledged" : true, "matchedCount" : 5, "modifiedCount" : 5 }

The following query returns all documents from the products collection:

db.products.find({}, {
    name: 1,
    "spec.ram": 1
})

Output:

{ "_id" : 1, "name" : "xPhone", "spec" : { "ram" : 8 } }
{ "_id" : 2, "name" : "xTablet", "spec" : { "ram" : 32 } }
{ "_id" : 3, "name" : "SmartTablet", "spec" : { "ram" : 24 } }
{ "_id" : 4, "name" : "SmartPad", "spec" : { "ram" : 16 } }
{ "_id" : 5, "name" : "SmartPhone", "spec" : { "ram" : 8 } }

Summary

• Use the MongoDB $mul operator to multiply the value of a field by a specified number.