$unwind 空数组 [英] $unwind empty array

问题描述

我有一个用户集合，其中每个文档都具有以下结构:

I have a collection of users where each document has following structure:

{
  "_id": "<id>",
  "login": "xxx",
  "solved": [
    {
      "problem": "<problemID>",
      "points": 10
    },
    ...
  ]
}

solved 字段可能为空或包含任意多个子文档.我的目标是获得一个用户列表以及总分(points 的总和)，其中尚未解决任何问题的用户将被分配总分 0.这可能吗?使用单个查询(最好使用聚合框架)?

The field solved may be empty or contain arbitrary many subdocuments. My goal is to get a list of users together with the total score (sum of points) where users that haven't solved any problem yet will be assigned total score of 0. Is this possible to do this with a single query (ideally using aggregation framework)?

我试图在聚合框架中使用以下查询:

I was trying to use following query in aggregation framework:

{ "$group": {
  "_id": "$_id",
  "login": { "$first": "$login" },
  "solved": { "$addToSet": { "points": 0 } }
} }
{ "$unwind": "$solved" }
{ "$group": {
  "_id": "$_id",
  "login": { "$first": "$login" },
  "solved": { "$sum": "$solved.points" }
} }

但是我收到以下错误:

exception: The top-level _id field is the only field currently supported for exclusion

提前致谢

推荐答案

在 MongoDB 3.2 及更新版本中，$unwind 运算符现在有一些选项，特别是 preserveNullAndEmptyArrays 选项将解决这个问题.

With MongoDB 3.2 version and newer, the $unwind operator now has some options where in particular the preserveNullAndEmptyArrays option will solve this.

如果此选项设置为 true 并且路径为空、缺失或空数组，$unwind 输出文档.如果为 false，$unwind如果路径为空、缺失或空数组，则不输出文档.在您的情况下，将其设置为 true:

If this option is set to true and if the path is null, missing, or an empty array, $unwind outputs the document. If false, $unwind does not output a document if the path is null, missing, or an empty array. In your case, set it to true:

db.collection.aggregate([
    { "$unwind": {
            "path": "$solved",
            "preserveNullAndEmptyArrays": true
    } },
    { "$group": {
        "_id": "$_id",
        "login": { "$first": "$login" },
        "solved": { "$sum": "$solved.points" }
    } }
])

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