MongoDB has a $mul field update operator that allows you to multiply a value by a specific amount.

If the field doesn’t already exist, it is created and its value is set to zero (0) using the same numeric type as the multiplier.

Example

Suppose we have a collection with the following document:

{ "_id" : 1, "bar" : 10 } 

We can use the $mul operator in conjunction with the update() method to increment the bar field.

Like this:

db.foo.update(
  { _id: 1 },
  { $mul: { bar: 2 } }
)

Output:

WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

This tells us that one document was matched and modified.

Let’s check the collection again:

db.foo.find()

Result:

{ "_id" : 1, "bar" : 20 } 

We can see that the amount has doubled to 20.

Multiply a Field that Doesn’t Exist

When you use $mul on a field that doesn’t exist in the document, the field is added and set to zero (0) using the same numeric type as the multiplier.

Example:

db.foo.update(
  { _id: 1 },
  { $mul: { bar: 3, extra: 2 } }
)

Output:

WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Notice that we updated two fields; the bar field and the extra field (which didn’t originally exist).

Let’s check the document again:

db.foo.find()

Result:

{ "_id" : 1, "bar" : 60, "extra" : 0 } 

We can see that the bar field has been multiplied by 3, and a new extra field has been added and set to 0.

Mixed Types

Multiplication with values of mixed numeric types (32-bit integer, 64-bit integer, float) may result in conversion of numeric type.

See the MongoDB documentation for an explanation.