How to Use MongoDB $inc Operator in update() Method

Summary: in this tutorial, you’ll learn how to use the $inc operator in the update() method to increment a field by a specified value.

Introduction to the MongoDB $inc operator

Sometimes, you need to increment the value of one or more fields by a specified value. In this case, you can use the update() method with the $inc operator.

The $inc operator has the following syntax:

{ $inc: {<field1>: <amount1>, <field2>: <amount2>, ...} }

In this syntax, the amount can be positive or negative. When it’s a positive value, the $inc increases the field by amount. If the amount is a negative value, then the $inc decreases the field by the absolute value of the amount.

If the field doesn’t exist, the $inc creates the field and sets the field to the specified amount.

MongoDB $inc 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 $inc operator to increase the value of a field

The following example uses the $inc operator to increase the price of the document _id 1 from the products collection by 50:

db.products.updateOne({
    _id: 1
}, {
    $inc: {
        price: 50
    }
})

It returned the following document indicating that one document matched and has been updated:

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

If you query the document _id 1, you’ll see that the price has been increased:

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

Output:

{ "_id" : 1, "name" : "xPhone", "price" : 849 }

2) Using the MongoDB $inc operator to decrease the value of a field

The following example uses the $inc operator to decrease the price by 150:

db.products.updateOne({
    _id: 1
}, {
    $inc: {
        price: -150
    }
})

Output:

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

The price of the document _id 1 has been decreased as shown in the output of the following query:

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

Output:

{ "_id" : 1, "name" : "xPhone", "price" : 699 }

3) Using MongoDB $inc operator to update values of multiple fields

The following example uses the $inc operator to increase the value of the price field as well as the ram field of the spec embedded document:

db.products.updateOne({
    _id: 1
}, {
    $inc: {
        price: 50,
        "spec.ram": 4
    }
})

Output:

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

Note that to specify a field in an embedded document, you use the dot notation e.g., "spec.ram".

This query selects the document _id 1 to verify the update:

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

The values of the price and ram fields have been increased as shown in the following output:

{ "_id" : 1, "name" : "xPhone", "price" : 749, "spec" : { "ram" : 8 } }

Summary

• Use the $inc operator to increase a value of a field by a specified amount.
• Use a negative value in the $inc operator to decrease the value of the field.