MongoDB $group by id where id name varies (conditionals?)-CodePudding

I need to group all entries by id using $group. However, the id is under different objects, so is $student.id for some and $teacher.id for others. I've tried $cond, $is and any other conditional I could find but haven't had any luck. What I'd want is something like this:

lessons.aggregate([
    // matches, lookups, etc.
    {$group: {
         "_id":{
                "id": (if student exists "student.id", else if teacher exists "teacher.id"
                },
         // other fields
        }
    }}]);

How can I do this? I've scoured the MongoDB docs for hours yet nothing works. I'm new to this company and trying to debug something so not familiar with the tech yet, so apologies if this is rudimentary stuff!

Update: providing some sample data to demo what I'd want. Shortened from the real thing to fit the question. After all the matches, lookups, etc and before using $group, the data looks like this. As student.id of first and second objects are the same, I want them to be grouped.

{
student: {
    _id: new ObjectId("61dc0fce904d07184b461c03"),
    name: Jess W
    },
duration: 30
},
{
student:{
    _id: new ObjectId("61dc0fce904d07184b461c03"),
    name: Jess W
    },
duration: 30
},
{
teacher: {
    _id: new ObjectId("61dc0f6a904d07184b461be7"),
    name: Michael S
    },
duration: 30
},
{
teacher: {
    _id: new ObjectId("61dc1087904d07184b461c6a"),
    name: Andrew J
    },
duration: 30
},

CodePudding user response：

You can just use $ifNull and chain them, like so:

db.collection.aggregate([
  {
    $group: {
      _id: {
        "$ifNull": [
          "$student._id",
          {
            $ifNull: [
              "$teacher._id",
              "$archer._id"
            ]
          }
        ]
      }
    }
  }
])

CodePudding user response：

If the fields exist "exclusive only", then you can simply combine them:

{ $group: {_id: {student: "$student.id", teacher: "$teacher.id"} }, // other fields }

concatenate them should also work:

{ $group: {_id: {$concat: [ "$student.id", "$teacher.id" ] }, // other fields }