Node.js MongoDB Aggregation

What is MongoDB Aggregation in Node.js?

The aggregation pipeline processes documents through stages — match, group, sort, project, lookup — and is how you compute totals, averages, groupings and joins inside MongoDB rather than in application code.

MongoDB Aggregation example

const stats = await Job.aggregate([
  { $match: { status: 'active', stipend: { $gt: 0 } } },   // filter FIRST
  { $group: {
      _id: '$city',
      avgStipend: { $avg: '$stipend' },
      maxStipend: { $max: '$stipend' },
      count: { $sum: 1 },
  } },
  { $match: { count: { $gte: 5 } } },
  { $sort: { avgStipend: -1 } },
  { $limit: 10 },
  { $project: { _id: 0, city: '$_id', avgStipend: { $round: ['$avgStipend', 0] }, count: 1 } },
]);

How this works

Putting $match first is the single most important optimisation: it uses indexes and shrinks the document set before the expensive grouping stage runs.

Key points to remember

• $match early, $project late — reduce documents before doing work on them.
• $lookup performs a left outer join with another collection.
• $unwind expands an array field into one document per element.
• Aggregation bypasses Mongoose schema casting — values come back raw.

Common mistakes with MongoDB Aggregation

• $match placed after $group, so the whole collection is grouped first.
• Pipelines that exceed the 100 MB memory limit without allowDiskUse.