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$setField

Definition

Sets a named field on an object copy.

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

Syntax

{
  "$setField": {
    "field": <field>,
    "input": <input>,
    "value": <value>
  }
}

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter Required Description
field Yes Expression supplying the field name to read or change.
input Yes Expression supplying the value to process. A field reference such as "$price" reads from the current document.
value Yes Expression supplying the value to evaluate.

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 field argument:

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

Argument handling

Invocation Result
{"$setField":[]} Error — $setField requires 'field', 'input', and 'value' keys.
{"$setField":{"unexpected":1}} Error — $setField requires 'field', 'input', and 'value' keys.

Examples

Expression: 

{
  "$setField": {
    "field": "a",
    "input": {},
    "value": 1
  }
}

Input document: 

{}

Observed SrotaX result: {"a":1}.

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: field

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

Argument: input

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

Argument: value

Input case Supplied value SrotaX result
null null {"a":null}
missing Field absent {"a":null}
zero 0 {"a":0}
negative -2 {"a":-2}
decimal 2.5 {"a":2.5}
numeric string "12" {"a":"12"}
text "hello" {"a":"hello"}
empty string "" {"a":""}
boolean true {"a":true}
false false {"a":false}
typed date {"$date":"2026-01-15T15:00:00Z"} {"a":{"$date":1768489200000}}
decimal128 {"$numberDecimal":"1.25"} {"a":{"$numberDecimal":"1.25"}}
empty array [] {"a":[]}
numeric array [1,2,3] {"a":[1,2,3]}
mixed array [1,null,"x",3] {"a":[1,null,"x",3]}
nested array [[1,2],[3]] {"a":[[1,2],[3]]}
empty object {} {"a":{}}
object {"x":1} {"a":{"x":1}}
ISO date string "2026-01-15T15:00:00Z" {"a":"2026-01-15T15:00:00Z"}
omitted named argument Argument removed Error — $setField requires 'field', 'input', and 'value' keys.

Verification

This page records 63 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: 21 exact BSON matches, 35 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
field / numeric string {"12":1} Error — Invalid $project :: caused by :: '$CURRENT.probe' is a field path reference which is not allowed in this context. Did you mean {$literal: '$CURRENT.probe'}?
field / text {"hello":1} Error — Invalid $project :: caused by :: '$CURRENT.probe' is a field path reference which is not allowed in this context. Did you mean {$literal: '$CURRENT.probe'}?
field / empty string {"":1} Error — Invalid $project :: caused by :: '$CURRENT.probe' is a field path reference which is not allowed in this context. Did you mean {$literal: '$CURRENT.probe'}?
field / ISO date string {"2026-01-15T15:00:00Z":1} Error — Invalid $project :: caused by :: '$CURRENT.probe' is a field path reference which is not allowed in this context. Did you mean {$literal: '$CURRENT.probe'}?
input / null Error — $setField requires document input and string field. null
input / missing Error — $setField requires document input and string field. null
value / missing {"a":null} {}

Download full comparison evidence.