Aggregation expressions can use both user-defined and system variables.
Variables can hold any BSON type data. To access the value of the variable, prefix the variable name 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. "$$<variable>.<field>".
User variable names can contain the ascii characters [_a-zA-Z0-9] and any non-ascii character.
User variable names must begin with a lowercase ascii letter [a-z] or a non-ascii character.
MongoDB offers the following system variables:
Variable
Description
NOW
A variable that returns the current datetime value. NOW returns the same value for all members of the deployment and remains the same throughout all stages of the aggregation pipeline.
CLUSTER_TIME
A variable that returns the current timestamp value.
CLUSTER_TIME is only available on replica sets and sharded clusters.
CLUSTER_TIME returns the same value for all members of the deployment and remains the same throughout all stages of the pipeline.
ROOT
References the root document, i.e. the top-level document, currently being processed in the aggregation pipeline stage.
CURRENT
References the start of the field path being processed in the aggregation pipeline stage. Unless documented otherwise, all stages start with CURRENT the same as ROOT.
CURRENT is modifiable. However, since $<field> is equivalent to $$CURRENT.<field>, rebinding CURRENT changes the meaning of $ accesses.
REMOVE
A variable which evaluates to the missing value. Allows for the exclusion of fields in $addFields and $project stages.
For examples that use $$REMOVE, see:
DESCEND
One of the allowed results of a $redact expression.
PRUNE
One of the allowed results of a $redact expression.
KEEP
One of the allowed results of a $redact expression.
SEARCH_META
A variable that stores the metadata results of an Atlas Search query. In all supported aggregation pipeline stages, a field set to the variable $$SEARCH_META returns the metadata results for the query.
For an example of its usage, see Atlas Search facet and count.
USER_ROLES
Returns the roles assigned to the current user.
For use cases that include USER_ROLES, see the find, aggregation, view, updateOne, updateMany, and findAndModify examples.
New in version 7.0.
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