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$mergeObjects

Definition

Combines objects with later values replacing earlier fields.

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

Syntax

{ "$mergeObjects": [ <object>, ... ] }

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter Required Description
object See argument count below Expression supplying an object to combine.

Arguments are positional: keep the order shown. Supply 1 or more arguments.

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 object argument:

Input case Supplied value Result
null null {"b":2}
missing Field absent {"b":2}
decimal 2.5 Error — $mergeObjects only accepts documents (objects).
numeric string "12" Error — $mergeObjects only accepts documents (objects).
text "hello" Error — $mergeObjects only accepts documents (objects).
boolean true Error — $mergeObjects only accepts documents (objects).
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $mergeObjects only accepts documents (objects).
empty array [] Error — $mergeObjects only accepts documents (objects).
numeric array [1,2,3] Error — $mergeObjects only accepts documents (objects).
object {"x":1} {"x":1,"b":2}
null null {"a":1}
missing Field absent {"a":1}
decimal 2.5 Error — $mergeObjects only accepts documents (objects).
numeric string "12" Error — $mergeObjects only accepts documents (objects).
text "hello" Error — $mergeObjects only accepts documents (objects).
boolean true Error — $mergeObjects only accepts documents (objects).
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $mergeObjects only accepts documents (objects).
empty array [] Error — $mergeObjects only accepts documents (objects).
numeric array [1,2,3] Error — $mergeObjects only accepts documents (objects).
object {"x":1} {"a":1,"x":1}

Argument handling

Invocation Result
{"$mergeObjects":[]} {}
{"$mergeObjects":{"unexpected":1}} {"unexpected":1}

Examples

Expression: 

{
  "$mergeObjects": [
    {
      "a": 1
    },
    {
      "b": 2
    }
  ]
}

Input document: 

{}

Observed SrotaX result: {"a":1,"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: object[0]

Input case Supplied value SrotaX result
null null {"b":2}
missing Field absent {"b":2}
zero 0 Error — $mergeObjects only accepts documents (objects).
negative -2 Error — $mergeObjects only accepts documents (objects).
decimal 2.5 Error — $mergeObjects only accepts documents (objects).
numeric string "12" Error — $mergeObjects only accepts documents (objects).
text "hello" Error — $mergeObjects only accepts documents (objects).
empty string "" Error — $mergeObjects only accepts documents (objects).
boolean true Error — $mergeObjects only accepts documents (objects).
false false Error — $mergeObjects only accepts documents (objects).
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $mergeObjects only accepts documents (objects).
decimal128 {"$numberDecimal":"1.25"} Error — $mergeObjects only accepts documents (objects).
empty array [] Error — $mergeObjects only accepts documents (objects).
numeric array [1,2,3] Error — $mergeObjects only accepts documents (objects).
mixed array [1,null,"x",3] Error — $mergeObjects only accepts documents (objects).
nested array [[1,2],[3]] Error — $mergeObjects only accepts documents (objects).
empty object {} {"b":2}
object {"x":1} {"x":1,"b":2}
ISO date string "2026-01-15T15:00:00Z" Error — $mergeObjects only accepts documents (objects).

Argument: object[1]

Input case Supplied value SrotaX result
null null {"a":1}
missing Field absent {"a":1}
zero 0 Error — $mergeObjects only accepts documents (objects).
negative -2 Error — $mergeObjects only accepts documents (objects).
decimal 2.5 Error — $mergeObjects only accepts documents (objects).
numeric string "12" Error — $mergeObjects only accepts documents (objects).
text "hello" Error — $mergeObjects only accepts documents (objects).
empty string "" Error — $mergeObjects only accepts documents (objects).
boolean true Error — $mergeObjects only accepts documents (objects).
false false Error — $mergeObjects only accepts documents (objects).
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $mergeObjects only accepts documents (objects).
decimal128 {"$numberDecimal":"1.25"} Error — $mergeObjects only accepts documents (objects).
empty array [] Error — $mergeObjects only accepts documents (objects).
numeric array [1,2,3] Error — $mergeObjects only accepts documents (objects).
mixed array [1,null,"x",3] Error — $mergeObjects only accepts documents (objects).
nested array [[1,2],[3]] Error — $mergeObjects only accepts documents (objects).
empty object {} {"a":1}
object {"x":1} {"a":1,"x":1}
ISO date string "2026-01-15T15:00:00Z" Error — $mergeObjects only accepts documents (objects).

Verification

This page records 41 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: 11 exact BSON matches, 30 both rejected, 0 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.

No differences were observed in the compared cases.

Download full comparison evidence.