$mul (field update operator)

APPLIES TO: MongoDB vCore

The $mul operator multiplies the value of a field by a specified number. If the field does not exist, $mul creates the field and sets it to zero. This operator is useful for applying percentage changes, scaling values, or performing bulk calculations on numeric fields.

Syntax

The syntax for the $mul operator is as follows:

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

Parameters

Description
field The name of the field to multiply.
number The number to multiply the field value by. Must be a numeric value.

Example

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

{
  "_id": "438db151-04b8-4422-aa97-acf94bc69cfc",
  "name": "Fourth Coffee | Turntable Boutique - Tromptown",
  "staff": {
    "totalStaff": {
      "fullTime": 8,
      "partTime": 5
    }
  },
  "sales": {
    "totalSales": 24863,
    "salesByCategory": [
      {
        "categoryName": "Direct-Drive Turntables",
        "totalSales": 24863
      }
    ]
  }
}

Example 1: Applying percentage increase

Apply a 10% increase to total sales (multiply by 1.1).

db.stores.updateOne(
  { "_id": "438db151-04b8-4422-aa97-acf94bc69cfc" },
  {
    $mul: {
      "sales.totalSales": 1.1
    }
  }
)

This will change totalSales from 24863 to 27349.3 (24863 × 1.1).

Example 2: Applying discount

Apply a 20% discount to sales figures (multiply by 0.8).

db.stores.updateOne(
  { "_id": "438db151-04b8-4422-aa97-acf94bc69cfc" },
  {
    $mul: {
      "sales.totalSales": 0.8
    }
  }
)

Example 3: Multiple field operations

Apply different multipliers to multiple fields simultaneously.

db.stores.updateOne(
  { "_id": "438db151-04b8-4422-aa97-acf94bc69cfc" },
  {
    $mul: {
      "staff.totalStaff.fullTime": 1.5,
      "staff.totalStaff.partTime": 2,
      "sales.totalSales": 1.25
    }
  }
)

This will:

  • Increase fullTime from 8 to 12 (8 × 1.5)
  • Increase partTime from 5 to 10 (5 × 2)
  • Increase totalSales by 25% (multiply by 1.25)

Example 4: Creating new fields

If a field doesn't exist, $mul creates it and sets it to 0.

db.stores.updateOne(
  { "_id": "438db151-04b8-4422-aa97-acf94bc69cfc" },
  {
    $mul: {
      "sales.bonusPoints": 100,
      "staff.overtimeHours": 40
    }
  }
)

Both bonusPoints and overtimeHours will be created with value 0.

Example 5: Working with decimals

Apply precise decimal multipliers for financial calculations.

db.stores.updateOne(
  { "_id": "438db151-04b8-4422-aa97-acf94bc69cfc" },
  {
    $mul: {
      "sales.totalSales": 0.915
    }
  }
)

Example 6: Updating array elements

Apply multipliers to specific elements within arrays using positional operators.

db.stores.updateOne(
  {
    "_id": "438db151-04b8-4422-aa97-acf94bc69cfc",
    "sales.salesByCategory.categoryName": "Direct-Drive Turntables"
  },
  {
    $mul: {
      "sales.salesByCategory.$.totalSales": 1.15
    }
  }
)

Example 7: Negative values and zero

Handle negative multipliers and zero values.

db.stores.updateOne(
  { "_id": "438db151-04b8-4422-aa97-acf94bc69cfc" },
  {
    $mul: {
      "sales.totalSales": -1,
      "staff.totalStaff.fullTime": 0
    }
  }
)

This will:

  • Make totalSales negative (useful for reversals)
  • Set fullTime to 0

After applying a 1.5 multiplier to staff and 1.25 to sales, the document would be updated as follows:

{
  "_id": "438db151-04b8-4422-aa97-acf94bc69cfc",
  "name": "Fourth Coffee | Turntable Boutique - Tromptown",
  "staff": {
    "totalStaff": {
      "fullTime": 12,
      "partTime": 10
    },
    "overtimeHours": 0
  },
  "sales": {
    "totalSales": 31078.75,
    "bonusPoints": 0,
    "salesByCategory": [
      {
        "categoryName": "Direct-Drive Turntables",
        "totalSales": 28592.45
      }
    ]
  }
}

Related content