如何用$lookup在Mongodb中统计名字

// 创建users集合
db.users.insertMany([
  { _id: 1, name: "Alice" },
  { _id: 2, name: "Bob" },
  { _id: 3, name: "Charlie" }
]);

// 创建orders集合，并关联users集合中的用户
db.orders.insertMany([
  { _id: 1, userId: 1, product: "Product A" },
  { _id: 2, userId: 1, product: "Product B" },
  { _id: 3, userId: 2, product: "Product C" },
  { _id: 4, userId: 3, product: "Product D" },
  { _id: 5, userId: 3, product: "Product E" }
]);

db.users.aggregate([
  {
    $lookup: {
      from: "orders",
      localField: "_id",
      foreignField: "userId",
      as: "orders"
    }
  },
  {
    $group: {
      _id: "$name",
      count: { $sum: 1 }
    }
  }
]);

[
  { "_id" : "Charlie", "count" : 2 },
  { "_id" : "Alice", "count" : 2 },
  { "_id" : "Bob", "count" : 1 }
]