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Variables in Aggregation Expressions

Aggregation expressions can use both user-defined and system variables.

Variables can hold any BSON type data. To access the value of the variable, use a string with the variable name prefixed with double dollar signs ($$); i.e. "$$<variable>".

If the variable references an object, to access a specific field in the object, use the dot notation; i.e. "$$<variable>.<field>".

User Variables

User variable names can contain the ascii characters [_a-zA-Z0-9] and any non-ascii character.

User variable names must begin with a lowercase ascii letter [a-z] or a non-ascii character.

System Variables

MongoDB offers the following system variables:

Variable Description
NOW

A variable the returns the current datetime value. NOW returns the same value for all members of the deployment and remains the same throughout all stages of the aggregation pipeline.

New in version 4.2.

CLUSTER_TIME

A variable the returns the current timestamp value.

CLUSTER_TIME is only available on replica sets and sharded clusters.

CLUSTER_TIME returns the same value for all members of the deployment and remains the same throughout all stages of the pipeline.

New in version 4.2.

ROOT
References the root document, i.e. the top-level document, currently being processed in the aggregation pipeline stage.
CURRENT

References the start of the field path being processed in the aggregation pipeline stage. Unless documented otherwise, all stages start with CURRENT the same as ROOT.

CURRENT is modifiable. However, since $<field> is equivalent to $$CURRENT.<field>, rebinding CURRENT changes the meaning of $ accesses.

REMOVE

A variable which evaluates to the missing value. Allows for the conditional exclusion of fields. In a $projection, a field set to the variable REMOVE is excluded from the output.

For an example of its usage, see Conditionally Exclude Fields.

New in version 3.6.

DESCEND
One of the allowed results of a $redact expression.
PRUNE
One of the allowed results of a $redact expression.
KEEP
One of the allowed results of a $redact expression.

See also

$let, $redact, $map