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$topN

Definition

Sort an array and return its first N items without changing the source.

Expression operator · Arrays · Studio Visual: available. Studio validates the complete expression separately.

Syntax

{
  "$topN": {
    "input": <input>,
    "sortBy": <sortBy>,
    "n": <n>
  }
}

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter Required Description
input Yes Expression supplying the value to process. A field reference such as "$price" reads from the current document.
sortBy Yes Literal 1 for ascending or -1 for descending; for object items, an ordered object of field names and directions.
n Yes Positive integer count of elements to return.

Behavior

The following observations use the example expression below. Each row substitutes one argument while retaining the other example arguments. A missing field differs from an explicit null; the result shown is what Core actually returned.

For the input argument:

Input case Supplied value Result
null null Error — $firstN input must be an array
missing Field absent Error — $firstN input must be an array
decimal 2.5 Error — $sortArray input must be an array
numeric string "12" Error — $sortArray input must be an array
text "hello" Error — $sortArray input must be an array
boolean true Error — $sortArray input must be an array
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $sortArray input must be an array
empty array [] []
numeric array [1,2,3] [3,2]
object {"x":1} Error — $sortArray input must be an array

Argument handling

Invocation Result
{"$topN":[]} Error — $topN requires input, sortBy and n
{"$topN":{"unexpected":1}} Error — $topN requires input, sortBy and n

Examples

Expression: 

{
  "$topN": {
    "input": [
      3,
      1,
      2
    ],
    "sortBy": -1,
    "n": 2
  }
}

Input document: 

{}

Observed SrotaX result: [3,2].

Detailed input cases

Expand the complete argument-by-argument test matrix

Each table changes only the indicated argument from the example above. Other arguments retain their baseline values. Values normally come from the input field probe; literal configuration arguments are replaced directly. “Missing” means the field is absent, not explicitly null. Numeric strings are strings, not numbers.

Argument: input

Input case Supplied value SrotaX result
null null Error — $firstN input must be an array
missing Field absent Error — $firstN input must be an array
zero 0 Error — $sortArray input must be an array
negative -2 Error — $sortArray input must be an array
decimal 2.5 Error — $sortArray input must be an array
numeric string "12" Error — $sortArray input must be an array
text "hello" Error — $sortArray input must be an array
empty string "" Error — $sortArray input must be an array
boolean true Error — $sortArray input must be an array
false false Error — $sortArray input must be an array
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $sortArray input must be an array
decimal128 {"$numberDecimal":"1.25"} Error — $sortArray input must be an array
empty array [] []
numeric array [1,2,3] [3,2]
mixed array [1,null,"x",3] ["x",3]
nested array [[1,2],[3]] [[3],[1,2]]
empty object {} Error — $sortArray input must be an array
object {"x":1} Error — $sortArray input must be an array
ISO date string "2026-01-15T15:00:00Z" Error — $sortArray input must be an array
omitted named argument Argument removed Error — $topN requires input, sortBy and n

Argument: sortBy

Input case Supplied value SrotaX result
null null Error — $sortArray sort direction must be 1 or -1
zero 0 Error — $sortArray sort direction must be 1 or -1
negative -2 Error — $sortArray sort direction must be 1 or -1
decimal 2.5 Error — $sortArray sort direction must be 1 or -1
numeric string "12" Error — $sortArray sort direction must be 1 or -1
text "hello" Error — $sortArray sort direction must be 1 or -1
empty string "" Error — $sortArray sort direction must be 1 or -1
boolean true Error — $sortArray sort direction must be 1 or -1
false false Error — $sortArray sort direction must be 1 or -1
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $sortArray sort direction must be 1 or -1
decimal128 {"$numberDecimal":"1.25"} Error — $sortArray sort direction must be 1 or -1
empty array [] Error — $sortArray sort direction must be 1 or -1
numeric array [1,2,3] Error — $sortArray sort direction must be 1 or -1
mixed array [1,null,"x",3] Error — $sortArray sort direction must be 1 or -1
nested array [[1,2],[3]] Error — $sortArray sort direction must be 1 or -1
empty object {} Error — $sortArray sortBy must not be empty
object {"x":1} [3,1]
ISO date string "2026-01-15T15:00:00Z" Error — $sortArray sort direction must be 1 or -1
omitted named argument Argument removed Error — $topN requires input, sortBy and n

Argument: n

Input case Supplied value SrotaX result
null null Error — $firstN n must be a positive 64-bit integer
missing Field absent Error — $firstN n must be a positive 64-bit integer
zero 0 Error — $firstN n must be a positive 64-bit integer
negative -2 Error — $firstN n must be a positive 64-bit integer
decimal 2.5 Error — $firstN n must be a positive 64-bit integer
numeric string "12" Error — $firstN n must be a positive 64-bit integer
text "hello" Error — $firstN n must be a positive 64-bit integer
empty string "" Error — $firstN n must be a positive 64-bit integer
boolean true Error — $firstN n must be a positive 64-bit integer
false false Error — $firstN n must be a positive 64-bit integer
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $firstN n must be a positive 64-bit integer
decimal128 {"$numberDecimal":"1.25"} Error — $firstN n must be a positive 64-bit integer
empty array [] Error — $firstN n must be a positive 64-bit integer
numeric array [1,2,3] Error — $firstN n must be a positive 64-bit integer
mixed array [1,null,"x",3] Error — $firstN n must be a positive 64-bit integer
nested array [[1,2],[3]] Error — $firstN n must be a positive 64-bit integer
empty object {} Error — $firstN n must be a positive 64-bit integer
object {"x":1} Error — $firstN n must be a positive 64-bit integer
ISO date string "2026-01-15T15:00:00Z" Error — $firstN n must be a positive 64-bit integer
omitted named argument Argument removed Error — $topN requires input, sortBy and n

Verification

This page records 62 executed cases. It covers individual argument substitutions, not every cross-product of arguments, locale, daylight-saving transitions, numeric overflow, collation or resource limits. No untested behavior is guaranteed.

Download exact expressions, inputs and SrotaX results. Errors are observed messages, not stable public error codes. The reference generator reads the checked-in execution report; do not edit generated observations by hand.

MongoDB differences

Compared with MongoDB 9.0.4: 6 exact BSON matches, 50 both rejected, 6 value/type/acceptance differences, 0 object-field-order differences.

“Exact” includes numeric BSON types and object key order. A value of 3 as int32 differs from 3.0 as double. Object-field-order differences are reported separately; object key ordering is not enforced by the engine regression baseline. Both rejected does not mean the error codes or messages match. This is finite-case evidence, not full compatibility certification.

Show observed differences
Tested case SrotaX MongoDB
input / null Error — $firstN input must be an array null
input / missing Error — $firstN input must be an array null
n / null Error — $firstN n must be a positive 64-bit integer null
n / missing Error — $firstN n must be a positive 64-bit integer null
n / zero Error — $firstN n must be a positive 64-bit integer []
sortBy / omitted named argument Error — $topN requires input, sortBy and n [1,2]

Download full comparison evidence.