$lookup多个级别

{ "_id": 1, "customerId": 101, "product": "Laptop" }
{ "_id": 2, "customerId": 102, "product": "Phone" }

{ "_id": 101, "name": "Alice", "countryId": 201 }
{ "_id": 102, "name": "Bob", "countryId": 202 }

{ "_id": 201, "name": "USA" }
{ "_id": 202, "name": "Canada" }

db.orders.aggregate([
  {
    $lookup: {
      from: "customers",
      localField: "customerId",
      foreignField: "_id",
      as: "customerInfo"
    }
  },
  {
    $unwind: "$customerInfo"
  },
  {
    $lookup: {
      from: "countries",
      localField: "customerInfo.countryId",
      foreignField: "_id",
      as: "countryInfo"
    }
  },
  {
    $unwind: "$countryInfo"
  },
  {
    $project: {
      _id: 1,
      product: 1,
      customerName: "$customerInfo.name",
      countryName: "$countryInfo.name"
    }
  }
])

{ "_id": 1, "product": "Laptop", "customerName": "Alice", "countryName": "USA" }
{ "_id": 2, "product": "Phone", "customerName": "Bob", "countryName": "Canada" }