将特定属性的文档限制为一定数量后汇总文档

Question

我需要找到所有用户的分数，只为每个用户挑选两个最好的分数，最后将这些分数相加来创建一个排行榜。

让我举个例子说明我正在寻找什么以及我已经尝试过但没有成功。

Users Collection:
[
  {user: "userA", score:10, game: 12},
  {user: "userA", score:20, game: 12},
  {user: "userA", score:7, game: 12},
  {user: "userB", score:10, game: 12},
  {user: "userB", score:3, game: 12},
  {user: "userB", score:7, game: 12}
]

我想要的结果是这样的：

[
  {user:"userA", score:30, game:12},
  {user:"userB", score:17, game:12}
]

我尝试过聚合的 mongo 方式，但不幸的是你不能 $sort, $limit 在聚合的 $group 阶段，这意味着你不能只选择两个最好的分数。

有没有办法通过处理简单的 find() 查询的返回文档，使用 mapReduce 甚至使用 Lodash 来做到这一点？

Answer 1

您可以使用lodash的_.sortBy()、_.groupBy()和_.map()得到每个用户的最高2个分数并将它们相加：

var arr = [
  {user: "userA", score:10, game: 12},
  {user: "userA", score:20, game: 12},
  {user: "userA", score:7, game: 12},
  {user: "userB", score:10, game: 12},
  {user: "userB", score:3, game: 12},
  {user: "userB", score:7, game: 12}
];

var result = _(arr) // start a lodash's chain
  .sortBy(function(value) { // sort the array descending
    return -value.score;
  })
  .groupBy('user') // group by the user
  .map(function(arr) { // map each user
    var score = arr[0].score + (arr[1] ? arr[1].score : 0); // the score should be a combination of the 1st two items (if there are two)
    return Object.assign({}, arr[0], { score, score }); // assign to new object, with the calculated score
  })
  .value(); // extract the result from the chain

console.log(result);

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