Variables in Aggregation Expressions
- User Variables
- System Variables

Variables in Aggregation Expressions

Aggregation expressions can use bothuser-defined and system variables.

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

If the variable references an object, to access a specific field in theobject, 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 thedeployment and remains the same throughout all stages of theaggregation pipeline.New in version 4.2.
- `CLUSTER_TIME`-	A variable that returns the current timestamp value.`CLUSTER_TIME` is only available on replica sets andsharded clusters.`CLUSTER_TIME` returns the same value for all membersof the deployment and remains the same throughout all stages ofthe pipeline.New in version 4.2.
- `ROOT`-	References the root document, i.e. the top-level document, currentlybeing processed in the aggregation pipeline stage.
- `CURRENT`-	References the start of the field path being processed in theaggregation pipeline stage. Unless documented otherwise, allstages start with `CURRENT` the same as`ROOT`.`CURRENT` is modifiable. However, since `$<field>`is equivalent to `$$CURRENT.<field>`, rebinding`CURRENT` changes the meaning of `$` accesses.
- `REMOVE`-	A variable which evaluates to the missing value. Allows for theconditional exclusion of fields. In a `$projection`, afield set to the variable `REMOVE` is excluded fromthe 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.