Release Notes for MongoDB 4.0 - Aggregation

[mongo]

New Type Conversion Operators

MongoDB 4.0 adds the following new aggregation operators for type conversion:

Operator	Description
$convert	Convert value to specified type.
$toBool	Convert value to boolean.
$toDate	Convert value to Date.
$toDecimal	Convert value to Decimal128.
$toDouble	Convert value to Double.
$toInt	Convert value to integer.
$toLong	Convert value to long.
$toObjectId	Convert value to ObjectId.
$toString	Convert value to string.

New String Operators

MongoDB 4.0 adds the following new aggregation string operators:

Operator	Description
$ltrim	Removes whitespace or the specified characters from the beginning of a string.
$rtrim	Removes whitespace or the specified characters from the end of a string.
$trim	Removes whitespace or the specified characters from the beginning and end of a string.

Additional Improvements

$bucket

The $bucket stage no longer requires boundaries document arguments to be wrapped in $literal.

$dateToString

The $dateToString aggregation operator has the following option changes:

NOTE

Requires featureCompatibilityVersion (fCV) set to "4.0" or greater.

• A new option onNull specifies the value to return if the date is null or missing.
• The option format is now optional.

$dateFromParts

If the value specified for fields other than year, isoYear, and timezone is outside the valid range, $dateFromParts carries or subtracts the difference from other date parts to calculate the date. For more information, see Value Range

$dateFromString

The $dateFromString aggregation operator takes an optional format field.

$currentOp

The aggregation pipeline stage $currentOp supports the following new options:

• idleSessions option to return information on inactive sessions which are holding locks as part of a transaction.
• localOps option to report operations that are running locally on the current mongos instance, rather than reporting operations that are running on the shards.