$mul

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:

Multiply the Value of a Field

Consider a collection products with the following document:

{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("10.99"), "qty" : 25 }

The following db.collection.update() operation updates the document, using the $mul operator to multiply the price by 1.25 and the qty field by 2:

db.products.update(
   { _id: 1 },
   { $mul: { price: NumberDecimal("1.25"), qty: 2 } }
)

The operation results in the following document, where the new value of price reflects the original value 10.99 multiplied by 1.25 and the new value of qty reflects the original value of 25 multipled by 2:

{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("13.7375"), "qty" : 50 }

Apply $mul Operator to a Non-existing Field

Consider a collection products with the following document:

{ _id: 2,  item: "Unknown" }

The following db.collection.update() operation updates the document, applying the $mul operator to the field price that does not exist in the document:

db.products.update(
   { _id: 2 },
   { $mul: { price: NumberLong(100) } }
)

The operation results in the following document with a price field set to value 0 of numeric type NumberLong, the same type as the multiplier:

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

Multiply Mixed Numeric Types

Consider a collection products with the following document:

{ _id: 3,  item: "XYZ", price: NumberLong(10) }

The following db.collection.update() operation uses the $mul operator to multiply the value in the price field NumberLong(10) by NumberInt(5):

db.products.update(
   { _id: 3 },
   { $mul: { price: NumberInt(5) } }
)

The operation results in the following document:

{ "_id" : 3, "item" : "XYZ", "price" : NumberLong(50) }

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