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Field Update Operators

$mul

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  • Definition
  • Behavior
  • Examples

Definition

$mul

Multiply the value of a field by a number. To specify a $mul expression, use the following prototype:

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

The field to update must contain a numeric value.

To specify a <field> in an embedded document or in an array, use dot notation.

Behavior

Starting in MongoDB 5.0, mongod no longer raises an error when you use an update operator like $mul with an empty operand expression ( { } ). An empty update results in no changes and no oplog entry is created (meaning that the operation is a no-op).

Starting in MongoDB 5.0, update operators process document fields with string-based names in lexicographic order. Fields with numeric names are processed in numeric order. See Update Operators Behavior for details.

Missing Field

If the field does not exist in a document, $mul creates the field and sets the value to zero of the same numeric type as the multiplier.

Atomic

$mul is an atomic operation within a single document.

Mixed Type

Multiplication with values of mixed numeric types (32-bit integer, 64-bit integer, float) may result in conversion of numeric type. For multiplication with values of mixed numeric types, the following type conversion rules apply:

32-bit Integer
64-bit Integer
Float
32-bit Integer
32-bit or 64-bit Integer
64-bit Integer
Float
64-bit Integer
64-bit Integer
64-bit Integer
Float
Float
Float
Float
Float

Note

  • If the product of two 32-bit integers exceeds the maximum value for a 32-bit integer, the result is a 64-bit integer.

  • Integer operations of any type that exceed the maximum value for a 64-bit integer produce an error.

Examples

Multiply the Value of a Field

Create the products collection:

db.products.insertOne(
   { "_id" : 1, "item" : "Hats", "price" : Decimal128("10.99"), "quantity" : 25 }
)

In the following operation, db.collection.updateOne() updates the document. The $mul operator multiplies the price field by 1.25 and the quantity field by 2:

db.products.updateOne(
   { _id: 1 },
   { $mul:
      {
         price: Decimal128( "1.25" ),
         quantity: 2
       }
   }
)

In the updated document:

  • price is the original value, 10.99, multiplied by 1.25

  • quantity is the original value, 25, multiplied by 2

{ _id: 1, item: 'Hats', price: Decimal128("13.7375"), quantity: 50 }

Apply $mul Operator to a Non-existing Field

Add the following document to the products collection:

db.products.insertOne( { _id: 2,  item: "Unknown" } )

In the following operation, db.collection.updateOne() attempts to apply the $mul operator to a field that is not in the document:

db.products.updateOne(
   { _id: 2 },
   { $mul: { price: Decimal128("100") } }
)

The db.collection.updateOne() operation

  • inserts the price field

  • sets Decimal128("0")

{ "_id" : 2, "item" : "Unknown", "price" : NumberLong(0) }

The price field has the same type, Decimal128, as the multiplier.

Multiply Mixed Numeric Types

Add the following document to the products collection:

db.products.insertOne( { _id: 3,  item: "Scarf", price: Decimal128("10") } )

In the following operation, db.collection.updateOne() uses the $mul operator to multiply the value in the price field Decimal128(10) by Int32(5):

db.products.updateOne(
   { _id: 3 },
   { $mul: { price: Int32(5) } }
)

The operation results in the following document:

{ _id: 3, item: 'Scarf', price: Decimal128("50") }

The value in the price field is of type Decimal128. See Multiplication Type Conversion Rules for details.

Tip

See also:

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