$ unwind空数组

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
    },
    ...
  ]
}

字段solved可以为空或包含任意多个子文档.我的目标是获取用户列表以及总得分(points的总和)，在该总得分中，尚未解决任何问题的用户将被分配总得分0.是否可以通过一个查询来做到这一点(理想情况下使用聚合框架)?

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" }
} }

但是我遇到以下错误:

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

提前谢谢

推荐答案

在MongoDB 3.2和更高版本中， 运算符现在具有一些选项，其中特别是preserveNullAndEmptyArrays选项将解决此问题.

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，缺少或为空数组，则 输出文档.如果为假，则 $unwind 不会输出文档路径为空，缺少或为空数组.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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