MongoDB - 计算对象属性嵌套数组的平均值

Question

我有一个包含嵌套球员的球队数据库，如下所示:

{
 team_id: "eng1",
 date_founded: new Date("Oct 04, 1896"),
 league: "Premier League",
 points: 62,
 name: "Manchester United",
 players: [ { p_id: "Rooney", goal: 85, caps: 125, age: 28 },
            { p_id: "Scholes", goal: 15, caps: 225, age: 28 },
            { p_id: "Giggs", goal: 45, caps: 359, age: 38 } ]
}

我正在尝试计算每支球队的平均年龄(所有球员年龄的平均值)，但我无法正确访问 $player.age 值。

cursor = db.teams.aggregate({ 
  $group : { _id: "$name", avgAge : { 
    $avg : "$players.age" }
  }
});

这只会返回以下内容:

{
  { "_id": "AC Milan", avgAge: 0 },
  { "_id": "Barcelona", avgAge: 0 }
  { "_id": "Real Madrid", avgAge: 0 }
  ...
}

(玩家年龄肯定都在)

有什么帮助吗？

Answer 1

因为 players 字段是一个数组，试图用 $players.age 访问它的成员太困惑了，mongo 不知道你是数组的哪个元素想访问。然后用$unwind来拯救，它会让数组的每个元素成为字段players的嵌入元素。如果您对文档“Manchester United”执行 $unwind，您将得到类似这样的内容

{ "_id" : ObjectId("5666fbbd755e59eab7a3e05e"), "team_id" : "eng1", "date_founded" : ISODate("1896-10-03T17:00:00Z"), "league" : "Premier League", "points" : 62, "name" : "Manchester United", "players" : { "p_id" : "Rooney", "goal" : 85, "caps" : 125, "age" : 28 } }
{ "_id" : ObjectId("5666fbbd755e59eab7a3e05e"), "team_id" : "eng1", "date_founded" : ISODate("1896-10-03T17:00:00Z"), "league" : "Premier League", "points" : 62, "name" : "Manchester United", "players" : { "p_id" : "Scholes", "goal" : 15, "caps" : 225, "age" : 28 } }
{ "_id" : ObjectId("5666fbbd755e59eab7a3e05e"), "team_id" : "eng1", "date_founded" : ISODate("1896-10-03T17:00:00Z"), "league" : "Premier League", "points" : 62, "name" : "Manchester United", "players" : { "p_id" : "Giggs", "goal" : 45, "caps" : 359, "age" : 38 } }`

上面文档的字段 players 有 3 个元素，所以你将有3个与原始文档具有相同属性的文档，但数组中的元素已上移成为播放器的嵌入文档。然后使用 $players.age 可以轻松访问玩家的年龄，因为它是一个嵌入式文档。

最终查询

cursor = db.teams.aggregate([
    { $unwind: "$players" },
    { $group : { _id: "$name", avgAge : {  $avg : "$players.age" } } }
]);