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$set (field update operator)

APPLIES TO: MongoDB vCore

The $set operator is used to set the value of a field in a document during update operations. If the field does not exist, $set will create the field with the specified value. If the field already exists, $set will replace the existing value with the new value. This is one of the most commonly used update operators in MongoDB.

Syntax

The syntax for the $set operator is as follows:

{
  $set: {
    <field1>: <value1>,
    <field2>: <value2>,
    ...
  }
}

Parameters

Description
field The name of the field to set. Can be a top-level field or use dot notation for nested fields.
value The value to assign to the field. Can be any valid BSON type.

Example

Let's understand the usage with sample json from stores dataset.

{
  "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
  "name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
  "location": {
    "lat": 70.1272,
    "lon": 69.7296
  },
  "staff": {
    "totalStaff": {
      "fullTime": 19,
      "partTime": 20
    }
  },
  "sales": {
    "totalSales": 151864,
    "salesByCategory": [
      {
        "categoryName": "Sound Bars",
        "totalSales": 2120
      },
      {
        "categoryName": "Home Theater Projectors",
        "totalSales": 45004
      }
    ]
  }
}

Example 1: Setting top-level fields

Suppose you want to update the store's total sales and add a new field for the store's status.

db.stores.updateOne(
  { "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74" },
  {
    $set: {
      "sales.totalSales": 160000,
      "status": "active",
      "lastUpdated": new Date()
    }
  }
)

After above operation, the document would be updated as follows:

{
  "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
  "name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
  "location": {
    "lat": 70.1272,
    "lon": 69.7296
  },
  "staff": {
    "totalStaff": {
      "fullTime": 19,
      "partTime": 20
    }
  },
  "sales": {
    "totalSales": 160000,
    "salesByCategory": [
      {
        "categoryName": "Sound Bars",
        "totalSales": 2120
      },
      {
        "categoryName": "Home Theater Projectors",
        "totalSales": 45004
      }
    ]
  },
  "status": "active",
  "lastUpdated": ISODate("2025-06-05T10:30:00.000Z")
}

Example 2: Setting nested fields

You can set values for nested fields using dot notation.

db.stores.updateOne(
  { "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74" },
  {
    $set: {
      "location.address": "123 Entertainment Blvd",
      "location.city": "East Linwoodbury",
      "staff.totalStaff.contractors": 5,
      "staff.manager": "John Smith"
    }
  }
)

After above operation, the document would be updated as follows:

{
  "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
  "name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
  "location": {
    "lat": 70.1272,
    "lon": 69.7296,
    "address": "123 Entertainment Blvd",
    "city": "East Linwoodbury"
  },
  "staff": {
    "totalStaff": {
      "fullTime": 19,
      "partTime": 20,
      "contractors": 5
    },
    "manager": "John Smith"
  },
  "sales": {
    "totalSales": 151864,
    "salesByCategory": [
      {
        "categoryName": "Sound Bars",
        "totalSales": 2120
      },
      {
        "categoryName": "Home Theater Projectors",
        "totalSales": 45004
      }
    ]
  }
}

Example 3: Setting array elements

You can set specific array elements using positional notation.

db.stores.updateOne(
  { 
    "_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
    "sales.salesByCategory.categoryName": "Sound Bars"
  },
  {
    $set: {
      "sales.salesByCategory.$.totalSales": 2500,
      "sales.salesByCategory.$.lastUpdated": new Date()
    }
  }
)

Example 4: Bulk set operations

You can update multiple documents using updateMany().

db.stores.updateMany(
  { "sales.totalSales": { $gt: 100000 } },
  {
    $set: {
      "category": "high-volume",
      "priority": 1,
      "reviewDate": new Date()
    }
  }
)

Important

If the field specified in $set does not exist, it will be created.

If the field already exists, its value will be replaced with the new value.

The $set operator can be used with any valid BSON data types including objects, arrays, and primitive types.

When using dot notation with arrays, you can use positional operators ($, $[], $[<identifier>]) to target specific array elements.

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