How To Use the 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>, ... } }
Code language: HTML, XML (xml)

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]}
 ])
Code language: JavaScript (javascript)

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 } })
Code language: PHP (php)

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

{
  acknowledged: true,
  insertedId: null,
  matchedCount: 1,
  modifiedCount: 1,
  upsertedCount: 0
}
Code language: CSS (css)

The following query verifies the update:

db.products.find({
    _id: 5
}, {
    name: 1,
    price: 1
})
Code language: CSS (css)

Output:

[ { _id: 5, name: 'SmartPhone', price: 658.9000000000001 } ]
Code language: CSS (css)

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
    }
})
Code language: PHP (php)

Output:

{
  acknowledged: true,
  insertedId: null,
  matchedCount: 1,
  modifiedCount: 1,
  upsertedCount: 0
}
Code language: CSS (css)

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 })
Code language: CSS (css)

Output:

{ _id: 1, name: 'xPhone', storage: [ 128, 256, 512 ] }
Code language: CSS (css)

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
    }
})
Code language: PHP (php)

Output:

{
  acknowledged: true,
  insertedId: null,
  matchedCount: 5,
  modifiedCount: 5,
  upsertedCount: 0
}
Code language: CSS (css)

The following query returns all documents from the products collection:

db.products.find({}, { name: 1, "spec.ram": 1 })
Code language: CSS (css)

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 } }
]
Code language: JavaScript (javascript)

Summary

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

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