MongoDB Foundations·Documents with discipline

The aggregation pipeline

Stages transform a stream of documents one step at a time.

Beginner 15 min read Lesson 9 of 300 Challenge included

What you will be able to do

Start with $match to reduce the working set.
Use $group with explicit accumulators such as $sum and $avg.
Inspect each stage with a small dataset before running at scale.

In plain English

$match filters early, $group aggregates, $lookup joins and $project shapes output. Put $match and $limit as early as possible so later stages process less data. Use $facet only when you need multiple summaries in one pass.

pipeline.jsJavaScript

db.lessons.aggregate([
  { $match: { published: true } },
  { $group: {
      _id: "$courseSlug",
      lessons: { $sum: 1 },
      minutes: { $sum: "$minutes" }
  } },
  { $sort: { minutes: -1 } },
  { $limit: 5 }
])

Your turn

Summarize lessons by course

Medium

Aggregate published lessons by courseSlug and return lesson count and total minutes.

JAVASCRIPT · starter

db.lessons.aggregate([])

Check your understanding before moving on.

Knowledge checkQuestion 1

Which stage groups documents by a key?

Select an answer to continue

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Lesson 9 of 300 · 0% of course