Variables in Aggregation Expressions¶

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User Variables
System Variables

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. New in version 4.2.
`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. 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.