Aggregation stages

Stages transform a list of documents. Expression operators calculate a value inside a document. The same name can have different syntax in these contexts.

These pages describe the 21 registered Core stages. They are not a list of Studio palette steps. For the authoring UI, use the Flow step reference. SrotaX normally processes one request document; use array expressions such as $map and $sortArray to transform its lists.

Each page includes a runnable pipeline, actual Core output, and implementation limits. The stage examples are separate from the 6,495-case expression parity suite.

Stage	Purpose
$match	Retains documents that satisfy a condition object.
$addFields	Adds or replaces fields while retaining other fields.
$set	Adds or replaces fields while retaining other fields.
$project	Builds each output document from explicitly selected or computed fields.
$unset	Removes named fields from each document.
$limit	Keeps the first N documents.
$skip	Drops the first N documents.
$sort	Orders documents by one or more fields.
$unwind	Produces one document for each element of an array field.
$replaceRoot	Replaces the entire document with an object expression.
$replaceWith	Replaces the entire document with an object expression.
$group	Combines documents sharing a grouping key and computes accumulator outputs.
$sortByCount	Counts documents per grouping value and sorts groups by descending count.
$bucket	Groups numeric values into explicit lower-inclusive, upper-exclusive ranges.
$bucketAuto	Divides the numeric value range into equal-width buckets.
$facet	Runs named child pipelines over the same input list and collects their results.
$lookup	Adds matching records from a host-provided in-memory dataset.
$densify	Inserts documents for missing numeric or date positions.
$fill	Replaces null or missing values using a value expression or interpolation.
$setWindowFields	Adds values computed over an ordered partition of documents.
$subPipeline	Executes inline child pipelines or resolves child references through the caller’s execution context.