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$getField

Definition

Reads a named field from an explicit object.

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

Syntax

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

Argument handling

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

Examples

Expression: 

{
  "$getField": {
    "field": "a",
    "input": {
      "a": 1
    }
  }
}

Input document: 

{}

Observed SrotaX result: 1 (int32).

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

Argument: input

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

Additional boundary examples

Show 1 tested examples

Expression: 

{
  "$getField": {
    "field": "b",
    "input": {
      "a": 1
    }
  }
}

Input: 

{}

SrotaX: null.

Verification

This page records 44 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: 1 exact BSON matches, 19 both rejected, 24 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 null Missing (field omitted)
field / text null Missing (field omitted)
field / empty string null Missing (field omitted)
field / ISO date string null Missing (field omitted)
input / null Error — $getField requires a document input and string field. null
input / missing Error — $getField requires a document input and string field. Missing (field omitted)
input / zero Error — $getField requires a document input and string field. Missing (field omitted)
input / negative Error — $getField requires a document input and string field. Missing (field omitted)
input / decimal Error — $getField requires a document input and string field. Missing (field omitted)
input / numeric string Error — $getField requires a document input and string field. Missing (field omitted)
input / text Error — $getField requires a document input and string field. Missing (field omitted)
input / empty string Error — $getField requires a document input and string field. Missing (field omitted)
input / boolean Error — $getField requires a document input and string field. Missing (field omitted)
input / false Error — $getField requires a document input and string field. Missing (field omitted)
input / typed date Error — $getField requires a document input and string field. Missing (field omitted)
input / decimal128 Error — $getField requires a document input and string field. Missing (field omitted)
input / empty array Error — $getField requires a document input and string field. Missing (field omitted)
input / numeric array Error — $getField requires a document input and string field. Missing (field omitted)
input / mixed array Error — $getField requires a document input and string field. Missing (field omitted)
input / nested array Error — $getField requires a document input and string field. Missing (field omitted)
input / empty object null Missing (field omitted)
input / object null Missing (field omitted)
input / ISO date string Error — $getField requires a document input and string field. Missing (field omitted)
all / boundary example null Missing (field omitted)

Download full comparison evidence.