MongoDB Aggregation Pipeline

The aggregation pipeline is a framework for data aggregation modeled on the concept of data processing pipelines. Documents enter a multi-stage pipeline that transforms the documents into an aggregated result.

Core Stages

$match

The $match stage filters documents, similar to a query. Always place $match as early as possible in the pipeline.

$group

The $group stage groups documents by a specified field and can apply accumulator expressions.

db.orders.aggregate([

{ $match: { status: "completed" } },

{ $group: { _id: "$userId", total: { $sum: "$amount" } } },

{ $sort: { total: -1 } }

])

$lookup

The $lookup stage performs a left outer join to another collection.

Performance Tips

Always use indexes on fields used in $match stages. Limit the number of documents early in the pipeline with $match and $limit.

Conclusion

Mastering the aggregation pipeline opens up incredibly powerful data analysis capabilities within MongoDB itself, often eliminating the need for application-level data processing.