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, 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 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 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: Remove Fields Conditionally Exclude Fields
`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 MongoDB 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. Using the `USER_ROLES` system variable in aggregations is not supported in M0 and Flex clusters. New in version 7.0.

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