(MongoDB) MongoDB 의 시스템 변수(System Variables)

System Variables

MongoDB offers the following system variables:

Variable	Description
`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 `$` accesses.
`DESCEND`	One of the allowed results of a `$redact` expression.
`PRUNE`	One of the allowed results of a `$redact` expression.
`KEEP`	One of the allowed results of a `$redact` expression.