Aggregation Framework

Introduction

The Aggregation Framework is MongoDB's data processing engine. It transforms documents through a pipeline of stages — filter, group, project, lookup, unwind, facet — to produce reports, analytics and reshaped data in a single round-trip.

Understanding the topic

• Each stage takes documents in, emits documents out.
• Stages compose top-down; output of one feeds the next.
• Aggregations can hit indexes and run in parallel across shards.
• Replaces app-side joining, grouping and reporting.

Syntax reference

db.orders.aggregate([
  { $match: { status: "paid" } },
  { $group: { _id: "$userId", total: { $sum: "$amount" } } },
  { $sort: { total: -1 } },
  { $limit: 10 }
]);

Real-world use

Daily revenue rollups, leaderboards, RFM segmentation — all idiomatic aggregations.