$convert (aggregation)

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Definition

Compatibility
Syntax
Behavior
Example

Definition

$convert: Converts a value to a specified type.

Compatibility

You can use $convert for deployments hosted in the following environments:

MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

Syntax

$convert has the following syntax:

{
 $convert:
     {
        input: <expression>,
        to: <type expression>,
        onError: <expression>,  // Optional.
        onNull: <expression>    // Optional.
      }
}

The $convert takes a document with the following fields:

Field

Description

input

The argument can be any valid expression. For more information on expressions, see Expression Operators.

to

The argument can be any valid expression that resolves to one of the following numeric or string identifiers:

String Identifier	Numeric Identifier	Notes
"double"	1	For more information on the conversion to double, see Converting to a Double.
"string"	2	For more information on the conversion to string, see Converting to a String.
"objectId"	7	For more information on the conversion to objectId, see Converting to an ObjectId.
"bool"	8	For more information on the conversion to boolean, see Converting to a Boolean.
"date"	9	For more information on the conversion to date, see Converting to a Date.
"int"	16	For more information on the conversion to integer, see Converting to an Integer.
"long"	18	For more information on the conversion to long, see Converting to a Long.
"decimal"	19	For more information on the conversion to decimal, see Converting to a Decimal.

onError

Optional. The value to return on encountering an error during conversion, including unsupported type conversions. The arguments can be any valid expression.

If unspecified, the operation throws an error upon encountering an error and stops.

onNull

Optional. The value to return if the input is null or missing. The arguments can be any valid expression.

If unspecified, $convert returns null if the input is null or missing.

In addition to $convert, MongoDB provides the following aggregation operators as shorthand when the default "onError" and "onNull" behavior is acceptable:

Behavior

Converting to a Boolean

The following table lists the input types that can be converted to a boolean:

Input Type	Behavior
Array	Returns true
Binary data	Returns true
Boolean	No-op. Returns the boolean value.
Code	Returns true
Date	Returns true
Decimal	Returns true if not zero Return false if zero
Double	Returns true if not zero Return false if zero
Integer	Returns true if not zero Return false if zero
JavaScript	Returns true
Long	Returns true if not zero Return false if zero
MaxKey	Returns true
MinKey	Returns true
Null	Returns the value specified for the `onNull` option. By default, returns null.
Object	Returns true
ObjectId	Returns true
Regular expression	Returns true
String	Returns true
Timestamp	Returns true

To learn more about data types in MongoDB, see BSON Types.

The following table lists some conversion to boolean examples:

Example

Results

{ input: true, to: "bool" }

true

{ input: false, to: "bool" }

false

{ input: 1.99999, to: "bool" }

true

{ input: Decimal128( "5" ), to: "bool" }

true

{ input: Decimal128( "0" ), to: "bool" }

false

{ input: 100, to: "bool" }

true

{
   input: ISODate( "2018-03-26T04:38:28.044Z" ),
   to: "bool"
}

true

{ input: "hello", to: "bool" }

true

{ input: "false", to: "bool" }

true

{ input: "", to: "bool" }

true

{ input: null, to: "bool" }

null

Tip

Converting to an Integer

The following table lists the input types that can be converted to an integer:

Input Type	Behavior
Boolean	Returns `0` for `false`. Returns `1` for `true`.
Double	Returns truncated value. The truncated double value must fall within the minimum and maximum value for an integer. You cannot convert a double value whose truncated value is less than the minimum integer value or is greater than the maximum integer value.
Decimal	Returns truncated value. The truncated decimal value must fall within the minimum and maximum value for an integer. You cannot convert a decimal value whose truncated value is less than the minimum integer value or is greater than the maximum integer value.
Integer	No-op. Returns the integer value.
Long	Returns the long value as an integer. The long value must fall within the minimum and maximum value for an integer. You cannot convert a long value that is less than the minimum integer value or is greater than the maximum integer value.
String	Returns the numerical value of the string as an integer. The string value must be a base ₁₀ integer (e.g. `"-5"`, `"123456"`) and fall within the minimum and maximum value for an integer. You cannot convert a string value of a float or decimal or non-base ₁₀ number (e.g. `"-5.0"`, `"0x6400"`) or a value that falls outside the minimum and maximum value for an integer.

The following table lists some conversion to integer examples:

Example

Results

{ input: true, to: "int" }

{ input: false, to: "int" }

{ input: 1.99999, to: "int" }

{ input: Decimal128( "5.5000" ), to: "int" }

{
   input: Decimal128( "9223372036000.000" ),
   to: "int"
}

Error

{ input: Long( "5000" ), to: "int" }

5000

{ input: Long( "922337203600" ), to: "int" }

Error

{ input: "-2", to: "int" }

-2

{ input: "2.5", to: "int" }

Error

{ input: null, to: "int" }

null

Tip

Converting to a Decimal

The following table lists the input types that can be converted to a decimal:

Input Type	Behavior
Boolean	Returns `Decimal128( "0" )` for `false`. Returns `Decimal128( "1" )` for `true`.
Double	Returns double value as a decimal.
Decimal	No-op. Returns the decimal.
Integer	Returns the int value as a decimal.
Long	Returns the long value as a decimal.
String	Returns the numerical value of the string as a decimal. The string value must be of a base ₁₀ numeric value (e.g. `"-5.5"`, `"123456"`). You cannot convert a string value of a non-base ₁₀ number (e.g. `"0x6400"`)
Date	Returns the number of milliseconds since the epoch that corresponds to the date value.

The following table lists some conversion to decimal examples:

Example

Results

{ input: true, to: "decimal" }

Decimal128("1")

{ input: false, to: "decimal" }

Decimal128("0")

{ input: 2.5, to: "decimal" }

Decimal128( "2.50000000000000" )

{ input: Int32( 5 ), to: "decimal" }

Decimal128("5")

{ input: Long( 10000 ), to: "decimal" }

Decimal128("10000")

{ input: "-5.5", to: "decimal" }

Decimal128("-5.5")

{
   input: ISODate( "2018-03-26T04:38:28.044Z" ),
   to: "decimal"
}

Decimal128("1522039108044")

Tip

Converting to a Double

The following table lists the input types that can be converted to a double:

Input Type	Behavior
Boolean	Returns NumberDouble(0) for `false`. Returns NumberDouble(1) for `true`.
Double	No-op. Returns the double.
Decimal	Returns the decimal value as a double. The decimal value must fall within the minimum and maximum value for a double. You cannot convert a decimal value whose value is less than the minimum double value or is greater than the maximum double value.
Integer	Returns the int value as a double.
Long	Returns the long value as a double.
String	Returns the numerical value of the string as a double. The string value must be of a base ₁₀ numeric value (e.g. `"-5.5"`, `"123456"`) and fall within the minimum and maximum value for a double. You cannot convert a string value of a non-base ₁₀ number (e.g. `"0x6400"`) or a value that falls outside the minimum and maximum value for a double.
Date	Returns the number of milliseconds since the epoch that corresponds to the date value.

The following table lists some conversion to double examples:

Example

Results

{ input: true, to: "double" }

{ input: false, to: "double" }

{ input: 2.5, to: "double" }

2.5

{ input: Int32( 5 ), to: "double" }

{ input: Long( "10000" ), to: "double" }

10000

{ input: "-5.5", to: "double" }

-5.5

{ input: "5e10", to: "double" }

50000000000

{
   input: ISODate( "2018-03-26T04:38:28.044Z" ),
   to: "double"
}

1522039108044

Tip

Converting to a Long

The following table lists the input types that can be converted to a long:

Input Type	Behavior
Boolean	Returns `0` for `false`. Returns `1` for `true`.
Double	Returns truncated value. The truncated double value must fall within the minimum and maximum value for a long. You cannot convert a double value whose truncated value is less than the minimum long value or is greater than the maximum long value.
Decimal	Returns truncated value. The truncated decimal value must fall within the minimum and maximum value for a long. You cannot convert a decimal value whose truncated value is less than the minimum long value or is greater than the maximum long value.
Integer	Returns the int value as a long.
Long	No-op. Returns the long value.
String	Returns the numerical value of the string. The string value must be of a base ₁₀ long (e.g. `"-5"`, `"123456"`) and fall within the minimum and maximum value for a long. You cannot convert a string value of a float or decimal or non-base ₁₀ number (e.g. `"-5.0"`, `"0x6400"`) or a value that falls outside the minimum and maximum value for a long.
Date	Converts the Date into the number of milliseconds since the epoch.

The following table lists some conversion to long examples:

Example

Results

{ input: true, to: "long" }

Long("1")

{ input: false, to: "long"  }

Long("0")

{ input: 2.5, to: "long"  }

Long("2")

{ input: Decimal128( "5.5000" ), to: "long" }

Long("5")

{
   input: Decimal128( "9223372036854775808.0" ),
   to: "long"
}

Error

{ input: Int32( 8 ), to: "long" }

Long("8")

{
   input: ISODate( "2018-03-26T04:38:28.044Z" ),
   to: "long"
}

Long("1522039108044")

{ input: "-2", to: "long" }

Long("-2")

{ input: "2.5", to: "long" }

Error

{ input: null, to: "long" }

null

Tip

Converting to a Date

The following table lists the input types that can be converted to a date:

Input Type	Behavior
Double	Returns a date that corresponds to the number of milliseconds represented by the truncated double value. Positive number corresponds to the number of milliseconds since Jan 1, 1970. Negative number corresponds to the number of milliseconds before Jan 1, 1970.
Decimal	Returns a date that corresponds to the number of milliseconds represented by the truncated decimal value. Positive number corresponds to the number of milliseconds since Jan 1, 1970. Negative number corresponds to the number of milliseconds before Jan 1, 1970.
Long	Returns a date that corresponds to the number of milliseconds represented by the long value. Positive number corresponds to the number of milliseconds since Jan 1, 1970. Negative number corresponds to the number of milliseconds before Jan 1, 1970.
String	Returns a date that corresponds to the date string. The string must be a valid date string, such as: "2018-03-03" "2018-03-03T12:00:00Z" "2018-03-03T12:00:00+0500"
ObjectId	Returns a date that corresponds to the timestamp of the ObjectId.
Timestamp	Returns a date that corresponds to the timestamp.

The following table lists some conversion to date examples:

Example

Results

{
   input: 120000000000.5,
   to: "date"
}

ISODate("1973-10-20T21:20:00.000Z")

{
   input: Decimal128( "1253372036000.50" ),
   to: "date"
}

ISODate("2009-09-19T14:53:56.000Z")

{
   input: Long( "1100000000000" ),
   to: "date
}

ISODate("2004-11-09T11:33:20.000Z")

{
   input:  Long( "-1100000000000" ),
   to: "date"
}

ISODate("1935-02-22T12:26:40.000Z")

{
   input: ObjectId( "5ab9c3da31c2ab715d421285" ),
   to: "date"
}

ISODate("2018-03-27T04:08:58.000Z")

{ input:  "2018-03-03", to: "date" }

ISODate("2018-03-03T00:00:00.000Z")

{
   input: "2018-03-20 11:00:06 +0500",
   to: "date"
}

ISODate("2018-03-20T06:00:06.000Z")

{ input: "Friday", to: "date" }

Error

{
   input: Timestamp( { t: 1637688118, i: 1 } ),
   to: "date"
}

ISODate("2021-11-23T17:21:58.000Z")

Tip

Converting to an ObjectId

The following table lists the input types that can be converted to an ObjectId:

Input Type	Behavior
String	Returns an ObjectId for the hexadecimal string of length 24. You cannot convert a string value that is not a hexadecimal string of length 24.

The following table lists some conversion to date examples:

Example

Results

{
   input: "5ab9cbfa31c2ab715d42129e",
   to: "objectId"
}

ObjectId("5ab9cbfa31c2ab715d42129e")

{
   input: "5ab9cbfa31c2ab715d42129",
   to: "objectId"
}

Error

Tip

Converting to a String

The following table lists the input types that can be converted to a string:

Input Type	Behavior
Boolean	Returns the boolean value as a string.
Double	Returns the double value as a string.
Decimal	Returns the decimal value as a string.
Integer	Returns the integer value as a string.
Long	Returns the long value as a string.
ObjectId	Returns the ObjectId value as a hexadecimal string..
String	No-op. Returns the string value.
Date	Returns the date as a string.

The following table lists some conversion to string examples:

Example

Results

{ input: true, to: "string" }

"true"

{ input: false, to: "string" }

"false"

{ input: 2.5, to: "string" }

"2.5"

{ input: Int32( 2 ), to: "string" }

"2"

{ input:  Long( 1000 ), to: "string" }

"1000"

{
   input: ObjectId( "5ab9c3da31c2ab715d421285" ),
   to: "string"
}

"5ab9c3da31c2ab715d421285"

{
   input:  ISODate( "2018-03-27T16:58:51.538Z" ),
   to: "string"
}

"2018-03-27T16:58:51.538Z"

Tip

Example

Create a collection orders with the following documents:

db.orders.insertMany( [
   { _id: 1, item: "apple", qty: 5, price: 10 },
   { _id: 2, item: "pie", qty: 10, price: Decimal128("20.0") },
   { _id: 3, item: "ice cream", qty: 2, price: "4.99" },
   { _id: 4, item: "almonds" },
   { _id: 5, item: "bananas", qty: 5000000000, price: Decimal128("1.25") }
] )

The following aggregation operation on the orders collection converts the price to a decimal:

// Define stage to add convertedPrice and convertedQty fields with
//    the converted price and qty values.
// If price or qty values are missing, the conversion returns a
//    value of decimal value or int value of 0.
// If price or qty values cannot be converted, the conversion returns
//    a string
priceQtyConversionStage = {
   $addFields: {
      convertedPrice: { $convert:
         {
            input: "$price",
            to: "decimal",
            onError: "Error",
            onNull: Decimal128("0")
         } },
      convertedQty: { $convert:
         {
            input: "$qty",
            to: "int",
            onError:{ $concat:
               [
                  "Could not convert ",
                  { $toString:"$qty" },
                  " to type integer."
               ]
            },
         onNull: Int32("0")
      } },
   }
};
totalPriceCalculationStage = {
   $project: { totalPrice: {
     $switch: {
        branches: [
          { case:
             { $eq: [ { $type: "$convertedPrice" }, "string" ] },
             then: "NaN"
          },
          { case:
             { $eq: [ { $type: "$convertedQty" }, "string" ] },
             then: "NaN"
          },
        ],
        default: { $multiply: [ "$convertedPrice", "$convertedQty" ] }
     }
} } };
db.orders.aggregate( [
   priceQtyConversionStage,
   totalPriceCalculationStage
])

The operation returns the following documents:

{ _id: 1, totalPrice: Decimal128("50") },
{ _id: 2, totalPrice: Decimal128("200.0") },
{ _id: 3, totalPrice: Decimal128("9.98") },
{ _id: 4, totalPrice: Decimal128("0") },
{ _id: 5, totalPrice: 'NaN' }

Note

These examples use mongosh. The default types are different in the legacy mongo shell.

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$convert (aggregation)

Definition

Compatibility

Syntax

Behavior

Converting to a Boolean

Tip

See also:

Converting to an Integer

Tip

See also:

Converting to a Decimal

Tip

See also:

Converting to a Double

Tip

See also:

Converting to a Long

Tip

See also:

Converting to a Date

Tip

See also:

Converting to an ObjectId

Tip

See also:

Converting to a String

Tip

See also:

Example

Note