$add (aggregation)

Definition

$add

Adds numbers together or adds numbers and a date. If one of the arguments is a date, $add treats the other arguments as milliseconds to add to the date.

The $add expression has the following syntax:

{ $add: [ <expression1>, <expression2>, ... ] }

The arguments can be any valid expression as long as they resolve to either all numbers or to numbers and a date. For more information on expressions, see Expression Operators.

Starting in MongoDB 6.1 you can optimize the $add operation. To improve performance, group references at the end of the argument list. For example,

$add: [ 1, 2, 3, '$a', '$b', '$c' ]

Behavior

When input types are mixed, $add promotes the smaller input type to the larger of the two. A type is considered larger when it represents a wider range of values. The order of numeric types from smallest to largest is: integer → long → double → decimal

The larger of the input types also determines the result type unless the operation overflows and is beyond the range represented by that larger data type. In cases of overflow, $add promotes the result according to the following order:

• If the larger input type is integer, the result type is promoted to long.

• If the larger input type is long, the result type is promoted to double.

• If the larger type is double or decimal, the overflow result is represented as + or - infinity. There is no type promotion of the result.

When mixing Date and non-integer operands, $add rounds the non-integer value to the nearest integer before performing the operation.

Examples

The following examples use a sales collection with the following documents:

{ "_id" : 1, "item" : "abc", "price" : 10, "fee" : 2, date: ISODate("2014-03-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "fee" : 1, date: ISODate("2014-03-01T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5,  "fee" : 0, date: ISODate("2014-03-15T09:00:00Z") }

db.sales.aggregate(
   [
     { $project: { item: 1, total: { $add: [ "$price", "$fee" ] } } }
   ]
)

{ "_id" : 1, "item" : "abc", "total" : 12 }
{ "_id" : 2, "item" : "jkl", "total" : 21 }
{ "_id" : 3, "item" : "xyz", "total" : 5 }

db.sales.aggregate(
   [
     { $project: { item: 1, billing_date: { $add: [ "$date", 3*24*60*60000 ] } } }
   ]
)

{ "_id" : 1, "item" : "abc", "billing_date" : ISODate("2014-03-04T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "billing_date" : ISODate("2014-03-04T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "billing_date" : ISODate("2014-03-18T09:00:00Z") }