$project controls which fields appear and can calculate new fields, rename fields, and construct nested output documents. In an inclusion-style projection, _id remains included by default unless explicitly excluded. Do not mix inclusion and exclusion modes except for the special handling of _id.

A dollar-prefixed field path such as "$status" references a field in the current pipeline document. Expressions can be nested to calculate totals. Keep any fields required by later stages in the projection.

db.orders.aggregate([
  {
    $project: {
      _id: 1,
      customerId: 1,
      status: 1,
      itemSubtotal: {
        $sum: {
          $map: {
            input: "$items",
            as: "item",
            in: { $multiply: ["$$item.quantity", "$$item.price"] }
          }
        }
      },
      shippingFee: { $ifNull: ["$shippingFee", 0] }
    }
  }
]);