$arrayToObject

Definition

Converts key/value entries into an object.

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

Syntax

{ "$arrayToObject": <expression> }

Replace angle-bracket placeholders with expressions or values.

Expression supplying the value to process. A field reference such as "$price" reads from the current document.

The value goes directly after the operator name; do not add an input wrapper.

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	null
missing	Field absent	null
decimal	2.5	Error — $arrayToObject expects an array of {k, v} objects.
numeric string	"12"	Error — $arrayToObject expects an array of {k, v} objects.
text	"hello"	Error — $arrayToObject expects an array of {k, v} objects.
boolean	true	Error — $arrayToObject expects an array of {k, v} objects.
typed date	{"$date":"2026-01-15T15:00:00Z"}	Error — $arrayToObject expects an array of {k, v} objects.
empty array	[]	{}
numeric array	[1,2,3]	Error — $arrayToObject elements must be objects with 'k' and 'v'.
object	{"x":1}	Error — $arrayToObject expects an array of {k, v} objects.

Argument handling

Invocation	Result
{"$arrayToObject":[]}	{}
{"$arrayToObject":{"unexpected":1}}	Error — $arrayToObject expects an array of {k, v} objects.

Examples

Expression:

{
  "$arrayToObject": {
    "$literal": [
      {
        "k": "a",
        "v": 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: input

Input case	Supplied value	SrotaX result
null	null	null
missing	Field absent	null
zero	0	Error — $arrayToObject expects an array of {k, v} objects.
negative	-2	Error — $arrayToObject expects an array of {k, v} objects.
decimal	2.5	Error — $arrayToObject expects an array of {k, v} objects.
numeric string	"12"	Error — $arrayToObject expects an array of {k, v} objects.
text	"hello"	Error — $arrayToObject expects an array of {k, v} objects.
empty string	""	Error — $arrayToObject expects an array of {k, v} objects.
boolean	true	Error — $arrayToObject expects an array of {k, v} objects.
false	false	Error — $arrayToObject expects an array of {k, v} objects.
typed date	{"$date":"2026-01-15T15:00:00Z"}	Error — $arrayToObject expects an array of {k, v} objects.
decimal128	{"$numberDecimal":"1.25"}	Error — $arrayToObject expects an array of {k, v} objects.
empty array	[]	{}
numeric array	[1,2,3]	Error — $arrayToObject elements must be objects with 'k' and 'v'.
mixed array	[1,null,"x",3]	Error — $arrayToObject elements must be objects with 'k' and 'v'.
nested array	[[1,2],[3]]	Error — Object key must be a string
empty object	{}	Error — $arrayToObject expects an array of {k, v} objects.
object	{"x":1}	Error — $arrayToObject expects an array of {k, v} objects.
ISO date string	"2026-01-15T15:00:00Z"	Error — $arrayToObject expects an array of {k, v} objects.

Verification

This page records 22 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: 4 exact BSON matches, 17 both rejected, 1 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
all / empty arguments	{}	Error — Invalid $project :: caused by :: Expression $arrayToObject takes exactly 1 arguments. 0 were passed in.

Download full comparison evidence.