MongoDB Aggregation Builder

An aggregation pipeline is a sequence of stages that process documents. Each stage transforms the data passing through it. Documents flow through the stages sequentially — the output of one stage becomes the input of the next. Common use cases: filtering, grouping, counting, joining collections, reshaping documents.

The _id in $group is the grouping key — it determines which documents are grouped together. Use a field reference like '$category' to group by a field, null to aggregate all documents into one group, or an object like { year: { $year: '$date' }, status: '$status' } to group by multiple fields.

Placing $match as the first stage is critical for performance — it filters documents before any other processing, allowing MongoDB to use indexes. A $match at the end filters after all previous stages have processed, which wastes CPU and memory. Always put $match as early as possible.

$lookup performs a left outer join with another collection. 'from' is the collection to join, 'localField' is the field in the current collection, 'foreignField' is the matching field in the 'from' collection, and 'as' is the array field added to each document with the matching documents.

Use $project to include only the fields you need (set to 1), exclude fields (set to 0), or add computed fields. Projecting early in the pipeline reduces document size, which speeds up subsequent stages and reduces memory usage. Only _id requires explicit exclusion (set to 0) when projecting includes.

No. All pipeline building and JSON generation happens in your browser. Nothing is sent to any server.