MongoDB Aggregation Pipeline Explained

What is the Aggregation Pipeline?

Aggregation processes documents through a series of stages — filtering, grouping, transforming — to compute results like totals and averages. Think of it as MongoDB's version of SQL GROUP BY but far more powerful.

A Simple Pipeline

db.orders.aggregate([
  { $match: { status: "completed" } },     // filter
  { $group: {
      _id: "$customerId",
      total: { $sum: "$amount" }            // sum per customer
  }},
  { $sort: { total: -1 } }                  // highest first
]);

Common Stages

Stage	Does
$match	Filter documents
$group	Group & aggregate
$sort	Order results
$project	Reshape fields
$lookup	Join collections

Joining with $lookup

db.orders.aggregate([
  { $lookup: {
      from: "users",
      localField: "userId",
      foreignField: "_id",
      as: "user"
  }}
]);

FAQs

When use aggregation vs find?

Use find for simple queries; aggregation for grouping, joins, and computed results. More in our MongoDB guides.

Is aggregation fast?

Yes, especially with indexes on $match fields placed early.