$lookup performs a left outer join. It attaches matching foreign documents in an array field, even when the data model expects one match. Use $unwind if you need one output document per match; use preserveNullAndEmptyArrays: true if unmatched source records must remain.

Indexes on foreign join fields can matter for performance. _id is indexed by default. Filter source documents early, limit unnecessary foreign fields, and inspect the execution plan for large joins.

db.orders.aggregate([
  { $match: { status: "delivered" } },
  {
    $lookup: {
      from: "customers",
      localField: "customerId",
      foreignField: "_id",
      as: "customer"
    }
  },
  { $project: { customerId: 1, status: 1, createdAt: 1, customer: 1 } }
]);