Basic syntax

db.collection.aggregate([stage1, stage2, stage3])

A pipeline is an ordered array of stages. A stage consumes the current pipeline documents and produces documents for the next stage. Moving $match before or after $group can change the meaning: before grouping it filters source records; after grouping it filters summaries.

If $project removes a field, later stages cannot use the removed field unless it is recreated. Use explain() to investigate the execution plan and statistics for important pipelines.

db.orders.aggregate([
  {
    $set: {
      subtotal: {
        $sum: {
          $map: {
            input: "$items",
            as: "item",
            in: { $multiply: ["$$item.quantity", "$$item.price"] }
          }
        }
      }
    }
  },
  { $match: { subtotal: { $gt: 5000 } } }
]);