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$setUnion

Definition

Returns distinct combined array values.

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

Syntax

{ "$setUnion": [ <array>, <other>, ... ] }

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter Required Description
array See argument count below Expression supplying the source array.
other See argument count below Expression supplying the other array for the set operation.

Arguments are positional: keep the order shown. Supply 0 or more arguments.

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 array argument:

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

Argument handling

Invocation Result
{"$setUnion":[]} []
{"$setUnion":{"unexpected":1}} Error — $setUnion expects an array of arrays.

Examples

Expression: 

{
  "$setUnion": [
    [
      1,
      2
    ],
    [
      2,
      3
    ]
  ]
}

Input document: 

{}

Observed SrotaX result: [1,2,3].

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: array[0]

Input case Supplied value SrotaX result
null null Error — $setUnion expects each argument to be an array.
missing Field absent Error — $setUnion expects each argument to be an array.
zero 0 Error — $setUnion expects each argument to be an array.
negative -2 Error — $setUnion expects each argument to be an array.
decimal 2.5 Error — $setUnion expects each argument to be an array.
numeric string "12" Error — $setUnion expects each argument to be an array.
text "hello" Error — $setUnion expects each argument to be an array.
empty string "" Error — $setUnion expects each argument to be an array.
boolean true Error — $setUnion expects each argument to be an array.
false false Error — $setUnion expects each argument to be an array.
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $setUnion expects each argument to be an array.
decimal128 {"$numberDecimal":"1.25"} Error — $setUnion expects each argument to be an array.
empty array [] [2,3]
numeric array [1,2,3] [1,2,3]
mixed array [1,null,"x",3] [1,null,"x",3,2]
nested array [[1,2],[3]] [[1,2],[3],2,3]
empty object {} Error — $setUnion expects each argument to be an array.
object {"x":1} Error — $setUnion expects each argument to be an array.
ISO date string "2026-01-15T15:00:00Z" Error — $setUnion expects each argument to be an array.

Argument: other[1]

Input case Supplied value SrotaX result
null null Error — $setUnion expects each argument to be an array.
missing Field absent Error — $setUnion expects each argument to be an array.
zero 0 Error — $setUnion expects each argument to be an array.
negative -2 Error — $setUnion expects each argument to be an array.
decimal 2.5 Error — $setUnion expects each argument to be an array.
numeric string "12" Error — $setUnion expects each argument to be an array.
text "hello" Error — $setUnion expects each argument to be an array.
empty string "" Error — $setUnion expects each argument to be an array.
boolean true Error — $setUnion expects each argument to be an array.
false false Error — $setUnion expects each argument to be an array.
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $setUnion expects each argument to be an array.
decimal128 {"$numberDecimal":"1.25"} Error — $setUnion expects each argument to be an array.
empty array [] [1,2]
numeric array [1,2,3] [1,2,3]
mixed array [1,null,"x",3] [1,2,null,"x",3]
nested array [[1,2],[3]] [1,2,[1,2],[3]]
empty object {} Error — $setUnion expects each argument to be an array.
object {"x":1} Error — $setUnion expects each argument to be an array.
ISO date string "2026-01-15T15:00:00Z" Error — $setUnion expects each argument to be an array.

Verification

This page records 41 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: 7 exact BSON matches, 27 both rejected, 7 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
array[0] / null Error — $setUnion expects each argument to be an array. null
array[0] / missing Error — $setUnion expects each argument to be an array. null
array[0] / mixed array [1,null,"x",3,2] [null,1,2,3,"x"]
array[0] / nested array [[1,2],[3],2,3] [2,3,[1,2],[3]]
other[1] / null Error — $setUnion expects each argument to be an array. null
other[1] / missing Error — $setUnion expects each argument to be an array. null
other[1] / mixed array [1,2,null,"x",3] [null,1,2,3,"x"]

Download full comparison evidence.