Aggregation pipeline — match, group, project, unwind

A pipeline processes a stream of documents in stages. Match filters, group aggregates, project shapes the results, and unwind splits an array into separate documents.

db.orders.aggregate([
  { $match: { status: 'paid' } },
  { $group: { _id: '$customerId', total: { $sum: '$amount' } } },
  { $sort: { total: -1, _id: 1 } },
  { $limit: 5 }
]);

To be precise: sorting by the total comes after the group; a limit before grouping would change what the result means. In the example, amount is the value of the whole order.

Pitfall: unwind can multiply documents and inflate a sum. Filtering early helps, but every stage still has to match the task's logic.