Mongoose 将额外的集合加入到两个聚合集合中

Question

This is a new requirement I need to implement in my personal project. I successfully joined two collections using aggregate ( previous question) . Now, I have a new collection that I need to join. Please see codes below:

Student model:

{
  fullname: { type: String, required: true },
  email: { type: String, required: true },
}

Exams model:

{
  test: { type: String, required: false },
  top10: [
    type: {
      studentId: { type: String, required: true },
      score: { type: Number, required: false },
    }
  ]
}

Aggregated collections:

db.exams.aggregate([
{ $unwind: "$top10" },
{
$lookup: {
  from: "students",
  localField: "top10.studentId",
  foreignField: "_id",
  as: "students"
}
},
{ $addFields: { students: { $arrayElemAt: ["$students", 0] } } },
{
$group: {
  _id: "$_id",
  test: { $first: "$test" },
  students: {
    $push: {
      studentId: "$top10.studentId",
      score: "$top10.score",
      fullname: "$students.fullname"
    }
  }
}
}
])

Now, the 3rd collection is called Teacher .

Teacher model:

{
  fullName: { type: String, required: false },
  studentId: { type: String, required: false }
}

Expected output should be:

{
 "test": "Sample Test #1",
 "students": [
        {
            "studentId": "5f22ef443f17d8235332bbbe",
            "fullname": "John Smith",
            "score": 11,
            "teacher": "Dr. Hofstadter"
            
        },
        {
            "studentId": "5f281ad0838c6885856b6c01",
            "fullname": "Erlanie Jones",
            "score": 9,
            "teacher": "Mr. Roberts"
        },
        {
            "studentId": "5f64add93dc79c0d534a51d0",
            "fullname": "Krishna Kumar",
            "score": 5,
            "teacher": "Ms. Jamestown"
        }
    ]
}

Appreciate kind of help. Thanks!

Answer 1

You can add these 2 more pipelines before the $group pipeline,

• $lookup join collection with teacher
• $addFields to get convert array to object
• $group to add teacher name in students array

  {
    $lookup: {
      from: "teacher",
      localField: "top10.studentId",
      foreignField: "studentId",
      as: "students.teacher"
    }
  },
  {
    $addFields: {
      "students.teacher": { $arrayElemAt: ["$students.teacher", 0] }
    }
  },
  {
    $group: {
      _id: "$_id",
      test: { $first: "$test" },
      students: {
        $push: {
          studentId: "$top10.studentId",
          score: "$top10.score",
          fullname: "$students.fullname",
          teacher: { $ifNull: ["$students.teacher.fullname", ""] }
        }
      }
    }
  }

Playground