Variables in Aggregation Expressions¶

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Aggregation expressions can use both user-defined and system variables.

Variables can hold any BSON type data. To access the value of the variable, use a string with the variable name prefixed with double dollar signs ($$).

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
`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.