How to groupby on a field value that is inside an array in MongoDB?

Question

I have student grading system which makes use of MongoDB. I have the following documents in MongoDB:

How do I get the number of students who got grade "A","B","C","D" for a particular subject. The "_id" for every subject remains the same.

{
  _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",
}

This is what I have tried and stuck, not sure what the next steps are !?


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

Expected Output:

Distribution of students scoring different grades for each subject (_id)

{
  "_id" : "60b21e47e5462929cab27a98",
  "A" : 12,
  "B" : 20,
  "C" : 8,
  "D" : 2
},
{
  "_id" : "60b21e47e5462929cab27a99",
  "A" : 5,
  "B" : 2,
  "C" : 8,
  "D" : 12
}

turivishal · Accepted Answer · 2021-07-31 09:32:13Z

$unwind deconstruct the studentGradingDetails array
$group by _id and grade and get total count
$group by only _id and construct the key-value pair array of object with grade and count
$arrayToObject convert above key-value pair array to object
$mergeObjects to merge above converted object and _id field you can add more fields if needed
$replaceRoot to replace above merge object to root

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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