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$convert

Definition

Converts input to a named target type with optional fallbacks.

Expression operator · Types and conversion · Studio Visual: available. Studio validates the complete expression separately.

Syntax

{
  "$convert": {
    "input": <input>,
    "to": <to>,
    "onError": <onError>,
    "onNull": <onNull>
  }
}

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter Required Description
input Yes Expression supplying the value to process. A field reference such as "$price" reads from the current document.
to Yes Target type name or numeric type code. See Supported values below; for example, "int" or 16.
onError No Fallback expression for a supported conversion/parsing failure. If omitted, that failure raises an error.
onNull No Fallback expression for null or missing input. If omitted, the null-input result is null.

Supported to target names: array, object, bool, double, string, date, int, long, decimal. Array/object conversion parses a JSON string with the matching root. onError supplies a conversion-failure fallback; onNull supplies a null/missing fallback. Native ObjectId and UUID authoring remain outside the supported SrotaX scope.

Supported values

to accepts a numeric BSON type code or a string type name. Names are case-sensitive: use "int" or "string", not "INT" or "STRING".

String target Numeric code Result
"double" 1 Floating-point number
"string" 2 String
"object" 3 Object parsed from a JSON object string, existing object inputs are rejected
"array" 4 Array parsed from a JSON array string, existing array inputs are rejected
"bool" 8 Boolean
"date" 9 Date
"int" 16 Signed 32-bit integer
"long" 18 Signed 64-bit integer
"decimal" 19 Decimal number

Core additionally implements "objectId" / 7, but native ObjectId authoring is outside SrotaX product scope. UUID is not a supported target.

For example, these first two expressions both convert "42" to an integer; the third converts 42 to a string: 

{
  "$convert": {
    "input": "42",
    "to": "int"
  }
}


{
  "$convert": {
    "input": "42",
    "to": 16
  }
}

{
  "$convert": {
    "input": 42,
    "to": "string"
  }
}

With a non-null input, an unsupported string target throws unless onError is supplied. Unsupported numeric codes throw before onError is evaluated. A null input returns onNull (or null) before checking the target; a null target with non-null input returns null.

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 2 (int32)
numeric string "12" 12 (int32)
text "hello" Error — $convert failed for target int
boolean true 1 (int32)
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $convert failed for target int
empty array [] Error — $convert failed for target int
numeric array [1,2,3] Error — $convert failed for target int
object {"x":1} Error — $convert failed for target int

Argument handling

Invocation Result
{"$convert":[]} Error — $convert requires input and to
{"$convert":{"unexpected":1}} Error — $convert requires input and to

Examples

Expression: 

{
  "$convert": {
    "input": "12",
    "to": "int"
  }
}

Input document: 

{}

Observed SrotaX result: 12 (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: input

Input case Supplied value SrotaX result
null null null
missing Field absent null
zero 0 0 (int32)
negative -2 -2 (int32)
decimal 2.5 2 (int32)
numeric string "12" 12 (int32)
text "hello" Error — $convert failed for target int
empty string "" Error — $convert failed for target int
boolean true 1 (int32)
false false 0 (int32)
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $convert failed for target int
decimal128 {"$numberDecimal":"1.25"} 1 (int32)
empty array [] Error — $convert failed for target int
numeric array [1,2,3] Error — $convert failed for target int
mixed array [1,null,"x",3] Error — $convert failed for target int
nested array [[1,2],[3]] Error — $convert failed for target int
empty object {} Error — $convert failed for target int
object {"x":1} Error — $convert failed for target int
ISO date string "2026-01-15T15:00:00Z" Error — $convert failed for target int
omitted named argument Argument removed Error — $convert requires input and to

Argument: to

Input case Supplied value SrotaX result
null null null
missing Field absent null
zero 0 Error — Unsupported BSON type code
negative -2 Error — Unsupported BSON type code
decimal 2.5 "12"
numeric string "12" Error — $convert failed for target 12
text "hello" Error — $convert failed for target hello
empty string "" Error — $convert failed for target
boolean true Error — $convert failed for target true
false false Error — $convert failed for target false
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $convert failed for target Thu Jan 15 10:00:00 EST 2026
decimal128 {"$numberDecimal":"1.25"} 12.0 (double)
empty array [] Error — $convert failed for target []
numeric array [1,2,3] Error — $convert failed for target [1, 2, 3]
mixed array [1,null,"x",3] Error — $convert failed for target [1, null, x, 3]
nested array [[1,2],[3]] Error — $convert failed for target [[1, 2], [3]]
empty object {} Error — $convert failed for target Document{{}}
object {"x":1} Error — $convert failed for target Document{{x=1}}
ISO date string "2026-01-15T15:00:00Z" Error — $convert failed for target 2026-01-15T15:00:00Z
omitted named argument Argument removed Error — $convert requires input and to

Argument: onError

Input case Supplied value SrotaX result
null null 12 (int32)
missing Field absent 12 (int32)
zero 0 12 (int32)
negative -2 12 (int32)
decimal 2.5 12 (int32)
numeric string "12" 12 (int32)
text "hello" 12 (int32)
empty string "" 12 (int32)
boolean true 12 (int32)
false false 12 (int32)
typed date {"$date":"2026-01-15T15:00:00Z"} 12 (int32)
decimal128 {"$numberDecimal":"1.25"} 12 (int32)
empty array [] 12 (int32)
numeric array [1,2,3] 12 (int32)
mixed array [1,null,"x",3] 12 (int32)
nested array [[1,2],[3]] 12 (int32)
empty object {} 12 (int32)
object {"x":1} 12 (int32)
ISO date string "2026-01-15T15:00:00Z" 12 (int32)
omitted named argument Argument removed 12 (int32)

Argument: onNull

Input case Supplied value SrotaX result
null null 12 (int32)
missing Field absent 12 (int32)
zero 0 12 (int32)
negative -2 12 (int32)
decimal 2.5 12 (int32)
numeric string "12" 12 (int32)
text "hello" 12 (int32)
empty string "" 12 (int32)
boolean true 12 (int32)
false false 12 (int32)
typed date {"$date":"2026-01-15T15:00:00Z"} 12 (int32)
decimal128 {"$numberDecimal":"1.25"} 12 (int32)
empty array [] 12 (int32)
numeric array [1,2,3] 12 (int32)
mixed array [1,null,"x",3] 12 (int32)
nested array [[1,2],[3]] 12 (int32)
empty object {} 12 (int32)
object {"x":1} 12 (int32)
ISO date string "2026-01-15T15:00:00Z" 12 (int32)
omitted named argument Argument removed 12 (int32)

Additional boundary examples

Show 40 tested examples

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "[1,2147483648,9223372036854775808,1.5,null,{\"x\":true}]"
}

SrotaX: [1,2147483648,9.223372036854776e+18,1.5,null,{"x":true}].

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": null
}

SrotaX: null.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": ""
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "null"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "123"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "true"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "[]"
}

SrotaX: [].

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "{}"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "[1,]"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "{\"x\":1,}"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "/*x*/ []"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "[] {}"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "{\"x\\u0000\":1}"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "{\"x\":\"a\\u0000b\"}"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": "[1e400]"
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": 123
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": []
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "array"
  }
}

Input: 

{
  "value": {}
}

SrotaX: Error — $convert failed for target array.

Expression: 

{
  "$convert": {
    "input": "broken",
    "to": "array",
    "onError": "fallback"
  }
}

Input: 

{}

SrotaX: "fallback".

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "{\"a\":1,\"a\":2,\"nested\":{\"$date\":\"2020\"},\"x\":[1,2]}"
}

SrotaX: {"a":2,"nested":{"$date":"2020"},"x":[1,2]}.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": null
}

SrotaX: null.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": ""
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "null"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "123"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "true"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "[]"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "{}"
}

SrotaX: {}.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "[1,]"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "{\"x\":1,}"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "/*x*/ []"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "[] {}"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "{\"x\\u0000\":1}"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "{\"x\":\"a\\u0000b\"}"
}

SrotaX: {"x":"a\u0000b"}.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": "[1e400]"
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": 123
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": []
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "$value",
    "to": "object"
  }
}

Input: 

{
  "value": {}
}

SrotaX: Error — $convert failed for target object.

Expression: 

{
  "$convert": {
    "input": "broken",
    "to": "object",
    "onError": "fallback"
  }
}

Input: 

{}

SrotaX: "fallback".

Expression: 

{
  "$convert": {
    "input": {
      "a": 1
    },
    "to": {
      "type": "binData",
      "subtype": 0
    }
  }
}

Input: 

{}

SrotaX: Error — $convert failed for target {type=binData, subtype=0}.

Expression: 

{
  "$convert": {
    "input": "ff",
    "to": "int",
    "base": 16
  }
}

Input: 

{}

SrotaX: Error — $convert failed for target int.

Verification

This page records 123 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: 61 exact BSON matches, 58 both rejected, 4 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
to / decimal "12" Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: In $convert, numeric 'to' argument is not an integer
to / decimal128 12.0 (double) Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: In $convert, numeric 'to' argument is not an integer
all / boundary example Error — $convert failed for target {type=binData, subtype=0} {"$binary":{"base64":"DAAAABBhAAEAAAAA","subType":"00"}}
all / boundary example Error — $convert failed for target int 255 (int32)

Download full comparison evidence.