Mongodb Metabase

Aggregation expressions can use bothuser-defined and system variables.

Variables can hold any BSON type data.To access the value of the variable, prefix the variable name withdouble dollar signs ($$); i.e. '$$<variable>'.

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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.

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User variable names must begin with a lowercase ascii letter [a-z]or a non-ascii character.

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System Variables¶

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MongoDB offers the following system variables:

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Description
A variable that 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.
`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.
`ROOT`¶	References the root document, i.e. the top-level document, currentlybeing processed in the aggregation pipeline stage.
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 `$` accesses.
A variable which evaluates to the missing value. Allows for theconditional exclusion of fields. In a `$project`, afield set to the variable `REMOVE` is excluded fromthe output. For an example of its usage, see Conditionally Exclude Fields.
`DESCEND`¶	One of the allowed results of a `$redact` expression.
One of the allowed results of a `$redact` expression.
One of the allowed results of a `$redact` expression.