MongoDB Field Update Operator - $inc

Description

In MongoDB, the $inc operator is used to increment the value of a field by a specified amount. The $inc operator adds as a new field when the specified field does not exist, and sets the field to the specified amount. The $inc accepts positive and negative value as an incremental amount.

Syntax:

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

Parameters:

Our database name is 'myinfo' and our collection name is "items". Here, is the collection bellow.

Sample collection "items"

{
        "_id" : 1,
        "item_id" : "I001",
        "comp_id" : "C001",
        "qty" : 25,
        "prate" : 30,
        "srate" : 35,
        "mrp" : 40
}
{
        "_id" : 2,
        "item_id" : "I001",
        "comp_id" : "C002",
        "qty" : 30,
        "prate" : 32,
        "srate" : 37,
        "mrp" : 40
}

Example of field update operator - $inc

If we want to increment the value of qty by 10 for the first matching document in the collection items for item_id is I001 the following mongodb statement can be used:

> db.items.update( { item_id: "I001" },{ $inc: { qty: 10 }});

Here in the above example-

• The qty for the first matching document for the condition item_id is I001 have been updated and the qty became 35 from 25 and the rest remains same.

To see the updated output -

> db.items.find().pretty();

N.B. find() method displays the documents in a non structured format but to display the results in a formatted way, the pretty() method can be used.

Output of the command

{
        "_id" : 1,
        "item_id" : "I001",
        "comp_id" : "C001",
        "qty" : 35,
        "prate" : 30,
        "srate" : 35,
        "mrp" : 40
}
{
        "_id" : 2,
        "item_id" : "I001",
        "comp_id" : "C002",
        "qty" : 30,
        "prate" : 32,
        "srate" : 37,
        "mrp" : 40
}

Example of $inc operator with multi:true

If we want to add a new field like tax% with a specific value 2 for all the documents in the collection items for item_id is I001, following mongodb queries can be used -

> db.items.update( { "item_id": "I001" }, { $inc: { "tax%": 2 } }, { multi: true } );

Here in the above example-

• A new field tax% which does not exists have been added with a value of 2 for all documents which match the condition item_id is I001. Here "multi : true" have used to update all documents which matching the specified condition.

To see the updated output -

> db.items.find().pretty();

N.B. find() method displays the documents in a non structured format but to display the results in a formatted way, the pretty() method can be used.

Output of the command:

{
        "_id" : 1,
        "comp_id" : "C001",
        "item_id" : "I001",
        "mrp" : 40,
        "prate" : 30,
        "qty" : 35,
        "srate" : 35,
        "tax%" : 2
}
{
        "_id" : 2,
        "comp_id" : "C002",
        "item_id" : "I001",
        "mrp" : 40,
        "prate" : 32,
        "qty" : 30,
        "srate" : 37,
        "tax%" : 2
}

Example of $inc operator on multiple fields

If we want to increment the tax% and mrp for all the matching documents against a condition the following mongodb command can be used -

> db.items.update( { "item_id": "I001" }, { $inc: { "tax%": 2,"mrp":-2 } }, { multi: true } );

Here in the above example-

• the $inc operator expression specifies 2 for tax% and -2 for mrp field to increase the value of tax% by 2 and decrease the mrp by 2 simultaneously. The "multi : true" have used to update all matching documents for the specified condition.

To see the updated output -

> db.items.find().pretty();

N.B. find() method displays the documents in a non structured format but to display the results in a formatted way, the pretty() method can be used.

Output of the command:

{
        "_id" : 1,
        "comp_id" : "C001",
        "item_id" : "I001",
        "mrp" : 38,
        "prate" : 30,
        "qty" : 35,
        "srate" : 35,
        "tax%" : 4
}
{
        "_id" : 2,
        "comp_id" : "C002",
        "item_id" : "I001",
        "mrp" : 38,
        "prate" : 32,
        "qty" : 30,
        "srate" : 37,
        "tax%" : 4
}

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