$slice (aggregation)

Definition

$slice

Returns a subset of an array.

$slice has one of two syntax forms:

The following syntax returns elements from either the start or end of the array:

{ $slice: [ <array>, <n> ] }

The following syntax returns elements from the specified position in the array:

{ $slice: [ <array>, <position>, <n> ] }

Operand	Description
<array>	Any valid expression as long as it resolves to an array.
<position>	Optional. Any valid expression as long as it resolves to an integer. If positive, $slice determines the starting position from the start of the array. If <position> is greater than the number of elements, the $slice returns an empty array. If negative, $slice determines the starting position from the end of the array. If the absolute value of the <position> is greater than the number of elements, the starting position is the start of the array.
<n>	Any valid expression as long as it resolves to an integer. If <position> is specified, <n> must resolve to a positive integer. If positive, $slice returns up to the first n elements in the array. If the <position> is specified, $slice returns the first n elements starting from the position. If negative, $slice returns up to the last n elements in the array. n cannot resolve to a negative number if <position> is specified.

For more information on expressions, see Expressions.

Behavior

{ $slice: [ [ 1, 2, 3 ], 1, 1 ] }

[ 2 ]

{ $slice: [ [ 1, 2, 3 ], -2 ] }

[ 2, 3 ]

{ $slice: [ [ 1, 2, 3 ], 15, 2 ] }

[  ]

{ $slice: [ [ 1, 2, 3 ], -15, 2 ] }

[ 1, 2 ]

Example

A collection named users contains the following documents:

{ "_id" : 1, "name" : "dave123", favorites: [ "chocolate", "cake", "butter", "apples" ] }
{ "_id" : 2, "name" : "li", favorites: [ "apples", "pudding", "pie" ] }
{ "_id" : 3, "name" : "ahn", favorites: [ "pears", "pecans", "chocolate", "cherries" ] }
{ "_id" : 4, "name" : "ty", favorites: [ "ice cream" ] }

The following example returns at most the first three elements in the favorites array for each user:

db.users.aggregate([
   { $project: { name: 1, threeFavorites: { $slice: [ "$favorites", 3 ] } } }
])

The operation returns the following results:

{ "_id" : 1, "name" : "dave123", "threeFavorites" : [ "chocolate", "cake", "butter" ] }
{ "_id" : 2, "name" : "li", "threeFavorites" : [ "apples", "pudding", "pie" ] }
{ "_id" : 3, "name" : "ahn", "threeFavorites" : [ "pears", "pecans", "chocolate" ] }
{ "_id" : 4, "name" : "ty", "threeFavorites" : [ "ice cream" ] }