如何对 MongoDB 中数组内的字段值进行分组？

Posted 2023-02-16

【中文标题】如何对 MongoDB 中数组内的字段值进行分组？

【英文标题】：How to groupby on a field value that is inside an array in MongoDB?

【发布时间】：2021-10-06 12:56:55

【问题描述】：

我有使用 MongoDB 的学生评分系统。我在 MongoDB 中有以下文档：

我如何获取特定subject 获得grade "A","B","C","D" 的学生人数。每个主题的“_id”保持不变。

  _id: "60b223541338467beaf3ae0d",
  studentName: "John Doe"
  studentGradingDetails: [
    
      _id: "60b21e47e5462929cab27a98",
      term: "semester-1",
      subject: "chemistry",
      grade: "A",
      createdAt: "2021-05-29T10:58:15.113Z",
    ,
    
      _id: "60b21e47e5462929cab27a99",
      term: "semester-2",
      subject: "computer_science",
      grade: "B",
      createdAt: "2021-05-29T10:58:15.113Z",
    ,
  ],
  createdAt: "2021-05-29T11:19:48.770Z",


  _id: "60b223541338467beaf3ae0e",
  studentName: "Will Smith"
  studentGradingDetails: [
    
      _id: "60b21e47e5462929cab27a98",
      term: "semester-1",
      subject: "chemistry",
      grade: "D",
      createdAt: "2021-05-29T10:58:15.113Z",
    ,
    
      _id: "60b21e47e5462929cab27a99",
      term: "semester-2",
      subject: "computer_science",
      grade: "A",
      createdAt: "2021-05-29T10:58:15.113Z",
    ,
  ],
  createdAt: "2021-05-29T11:19:48.770Z",

这是我尝试并坚持的，不知道接下来的步骤是什么！？

await db.collection("studentsemestergrades")
        .aggregate([
             $unwind: "$studentGradingDetails" ,
            
                $group: 
                    _id: "$studentGradingDetails._id",
                    
                ,
            ,
        ])
        .toArray();

预期输出：

每个科目得分不同的学生分布(_id)

  "_id" : "60b21e47e5462929cab27a98",
  "A" : 12,
  "B" : 20,
  "C" : 8,
  "D" : 2
,

  "_id" : "60b21e47e5462929cab27a99",
  "A" : 5,
  "B" : 2,
  "C" : 8,
  "D" : 12

【参考方案1】：

$unwind 解构 studentGradingDetails 数组 $group by _id 和 grade 并获得总数 $group 仅由_id 和grade 和count 构造对象的键值对数组 $arrayToObject将上面的键值对数组转换为对象 $mergeObjects 合并以上转换后的对象和 _id 字段，如果需要，您可以添加更多字段 $replaceRoot 将上面的合并对象替换为根

await db.collection("studentsemestergrades").aggregate([
   $unwind: "$studentGradingDetails" ,
  
    $group: 
      _id: 
        _id: "$studentGradingDetails._id",
        grade: "$studentGradingDetails.grade"
      ,
      count:  $sum: 1 
    
  ,
  
    $group: 
      _id: "$_id._id",
      grades: 
        $push: 
          k: "$_id.grade",
          v: "$count"
        
      
    
  ,
  
    $replaceRoot: 
      newRoot: 
        $mergeObjects: [
           _id: "$_id" ,
           $arrayToObject: "$grades" 
        ]
      
    
  
])

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