MongoDB-计算对象属性嵌套数组的平均值 [英] MongoDB - calculating average of nested array of objects' attributes

问题描述

我有一个包含嵌套玩家的球队的数据库，如下所示:

I have a database of teams with nested players as below:

{
 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)，但是我无法正确访问$ player.age值.

I'm trying to calculate the average age of each team (the average of all players' ages), however I can't access the $player.age values correctly.

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 }
  ...
}

(所有年龄段的球员肯定都在那儿)

(The players ages are all definitely there)

有帮助吗?

推荐答案

由于players字段是一个数组，因此尝试使用$players.age访问其成员太混乱了，mongo不知道该数组的哪个元素您要访问. 然后$unwind进行救援，它将使数组的每个元素成为字段players的嵌入元素. 如果您对文档曼彻斯特联队"进行$unwind，您将拥有类似的内容

Since the players field is an array, trying to access its member with $players.age is too confusing, mongo doesn't know which element of the array you want to access. Then come $unwind to the rescue, it will make each element of the array to become the embedded element of the field players. If you do $unwind with the document "Manchester United", you will have something like this

{ "_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 } }`

上述文档的字段players具有3个元素，因此您将拥有 与原始文档具有相同属性的3个文档，但数组中的元素已上移为players的嵌入式文档. 然后，使用$players.age即可轻松了解播放器的年龄，因为它是嵌入式文档.

The field players of the above document have 3 element, so you will have 3 documents with same property like the original document, but the element in the array have moved up to be come embedded document of players. Then access to the age of a player is easy with $players.age because its an embedded document.

最终查询

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

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