MongoDB聚合索引在实际开发中的应用场景-嵌套文档的聚合查询

db.users.createIndex({ "user_id": 1, "orders.order_date": -1 })

db.users.aggregate([
  {
    $unwind: "$orders"
  },
  {
    $sort: { "user_id": 1, "orders.order_date": -1 }
  },
  {
    $group: {
      _id: "$user_id",
      name: { $first: "$name" },
      order_id: { $first: "$orders.order_id" },
      order_date: { $first: "$orders.order_date" },
      total_amount: { $first: "$orders.total_amount" }
    }
  },
  {
    $project: {
      _id: 0,
      user_id: "$_id",
      name: 1,
      order_id: 1,
      order_date: 1,
      total_amount: 1
    }
  }
])