$unsetField

Definition

Removes a named field from an object copy.

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

Syntax

{
  "$unsetField": {
    "field": <field>,
    "input": <input>
  }
}

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.

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

Argument handling

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

Examples

Expression:

{
  "$unsetField": {
    "field": "a",
    "input": {
      "a": 1,
      "b": 2
    }
  }
}

Input document:

{}

Observed SrotaX result: {"b":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: field

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

Argument: input

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

Verification

This page records 43 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: 3 exact BSON matches, 34 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
field / numeric string	{"a":1,"b":2}	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	{"a":1,"b":2}	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	{"a":1,"b":2}	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	{"a":1,"b":2}	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 — $unsetField requires document input and string field.	null
input / missing	Error — $unsetField requires document input and string field.	null

Download full comparison evidence.