$unwind expands an array so each element produces a separate pipeline document. If an order contains three items, unwinding items produces three documents for that order.

By default, documents with null, missing, or empty arrays do not produce output from $unwind. Use preserveNullAndEmptyArrays: true to retain the parent document in those cases. includeArrayIndex can capture an element's position.

Unwinding can multiply the number of documents processed. Use it when downstream stages need individual array elements; use $map, $filter, or $reduce when the calculation can remain inside the parent document.

db.orders.aggregate([
  { $match: { status: "delivered" } },
  { $unwind: "$items" },
  {
    $project: {
      orderId: "$_id",
      customerId: 1,
      productId: "$items.productId",
      productName: "$items.name",
      quantity: "$items.quantity",
      unitPrice: "$items.price",
      lineRevenue: { $multiply: ["$items.quantity", "$items.price"] }
    }
  }
]);