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$filter

Definition

Keeps array elements satisfying a boolean condition.

Expression operator · Arrays · Studio Visual: not exposed. Studio validates the complete expression separately.

Syntax

{
  "$filter": {
    "input": <input>,
    "as": <as>,
    "cond": <cond>
  }
}

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.
as No Literal variable name for the current array item; defaults to "this". Refer to it as $$name in the nested expression.
cond Yes Condition evaluated for each array item. Items with a matching condition are retained.

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 []
missing Field absent []
decimal 2.5 []
numeric string "12" []
text "hello" []
boolean true []
typed date {"$date":"2026-01-15T15:00:00Z"} []
empty array [] []
numeric array [1,2,3] [2,3]
object {"x":1} []

Argument handling

Invocation Result
{"$filter":[]} Error — $filter expects an object
{"$filter":{"unexpected":1}} []

Examples

Expression: 

{
  "$filter": {
    "input": [
      1,
      2
    ],
    "as": "x",
    "cond": {
      "$gt": [
        "$$x",
        1
      ]
    }
  }
}

Input document: 

{}

Observed SrotaX result: [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 []
missing Field absent []
zero 0 []
negative -2 []
decimal 2.5 []
numeric string "12" []
text "hello" []
empty string "" []
boolean true []
false false []
typed date {"$date":"2026-01-15T15:00:00Z"} []
decimal128 {"$numberDecimal":"1.25"} []
empty array [] []
numeric array [1,2,3] [2,3]
mixed array [1,null,"x",3] ["x",3]
nested array [[1,2],[3]] [[1,2],[3]]
empty object {} []
object {"x":1} []
ISO date string "2026-01-15T15:00:00Z" []
omitted named argument Argument removed []

Argument: as

Input case Supplied value SrotaX result
null null Error — Cannot invoke "Object.toString()" because the return value of "java.util.Map.get(Object)" is null
zero 0 Error — Undefined variable: $$x
negative -2 Error — Undefined variable: $$x
decimal 2.5 Error — Undefined variable: $$x
numeric string "12" Error — Undefined variable: $$x
text "hello" Error — Undefined variable: $$x
empty string "" Error — Undefined variable: $$x
boolean true Error — Undefined variable: $$x
false false Error — Undefined variable: $$x
typed date {"$date":"2026-01-15T15:00:00Z"} Error — Undefined variable: $$x
decimal128 {"$numberDecimal":"1.25"} Error — Undefined variable: $$x
empty array [] Error — Undefined variable: $$x
numeric array [1,2,3] Error — Undefined variable: $$x
mixed array [1,null,"x",3] Error — Undefined variable: $$x
nested array [[1,2],[3]] Error — Undefined variable: $$x
empty object {} Error — Undefined variable: $$x
object {"x":1} Error — Undefined variable: $$x
ISO date string "2026-01-15T15:00:00Z" Error — Undefined variable: $$x
omitted named argument Argument removed Error — Undefined variable: $$x

Argument: cond

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

Additional boundary examples

Show 2 tested examples

Expression: 

{
  "$filter": {
    "input": [
      1,
      2,
      3
    ],
    "as": "item",
    "cond": true,
    "limit": 1
  }
}

Input: 

{}

SrotaX: [1,2,3].

Expression: 

{
  "$filter": {
    "input": null,
    "cond": true
  }
}

Input: 

{}

SrotaX: [].

Verification

This page records 64 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: 10 exact BSON matches, 20 both rejected, 34 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 [] null
input / missing [] null
input / zero [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not int
input / negative [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not int
input / decimal [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not double
input / numeric string [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not string
input / text [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not string
input / empty string [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not string
input / boolean [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not bool
input / false [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not bool
input / typed date [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not date
input / decimal128 [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not decimal
input / empty object [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not object
input / object [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not object
input / ISO date string [] Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: input to $filter must be an array not string
cond / negative [] [1,2]
cond / decimal [] [1,2]
cond / numeric string [] [1,2]
cond / text [] [1,2]
cond / empty string [] [1,2]
cond / typed date [] [1,2]
cond / decimal128 [] [1,2]
cond / empty array [] [1,2]
cond / numeric array [] [1,2]
cond / mixed array [] [1,2]
cond / nested array [] [1,2]
cond / empty object [] [1,2]
cond / object [] [1,2]
cond / ISO date string [] [1,2]
input / omitted named argument [] Error — Invalid $project :: caused by :: Missing 'input' parameter to $filter
cond / omitted named argument [] Error — Invalid $project :: caused by :: Missing 'cond' parameter to $filter
all / unknown option [] Error — Invalid $project :: caused by :: Unrecognized parameter to $filter: unexpected
all / boundary example [1,2,3] [1]
all / boundary example [] null

Download full comparison evidence.