I am getting array of objects but I need objects in a single array by mongodb query

Question

Hi I am new to mongodb trying to achieve a output using mongodb aggregate I am trying to achieve a single array of objects instead of multiple array of objects can anyone help me to achieve my desired output

Below I have mentioned my model and I have mentioned my query, I am getting array of objects but I need a combined single array of objects I need to achieve it by mongodb query.

This is my mongodb model

db={
  "teacher": [
    {
      "rating": 4,
      "teacher_id": "123",
      "course_count": 1,
      
    },
    {
      "rating": 5,
      "teacher_id": "456",
      "course_count": 4,
      
    },
    {
      "rating": 5,
      "teacher_id": "no-exists",
      "course_count": 4,
      
    }
  ],
  "course": [
    {
      "teacher_id": "123",
      "cid": "1",
      "advanced": true,
      
    },
    {
      "teacher_id": "456",
      "cid": "1",
      "advanced": true,
      
    }
  ]
}

this is my mongodb query

db.course.aggregate([
  {
    "$match": {
      "cid": "1"
    }
  },
  {
    "$lookup": {
      "localField": "teacher_id",
      "from": "teacher",
      "foreignField": "teacher_id",
      "as": "teacher"
    }
  },
  {
    $unwind: {
      path: "$teacher",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $project: {
      _id: 0,
      cid: 1,
      teacher: 1
    }
  },
  
])

I am getting this output

[
  {
    "cid": "1",
    "teacher": {
      "_id": ObjectId("5a934e000102030405000002"),
      "course_count": 1,
      "rating": 4,
      "teacher_id": "123"
    }
  },
  {
    "cid": "1",
    "teacher": {
      "_id": ObjectId("5a934e000102030405000003"),
      "course_count": 4,
      "rating": 5,
      "teacher_id": "456"
    }
  }
]

this is my desired output


  {
    "cid": "1",
    "advance":true
    "teacher": [{
      "_id": ObjectId("5a934e000102030405000002"),
      "course_count": 1,
      "rating": 4,
      "teacher_id": "123"
    },
   {
      "_id": ObjectId("5a934e000102030405000003"),
      "course_count": 4,
      "rating": 5,
      "teacher_id": "456"
    }
]
}

Please take this with some caution, as I have not used MongoDB before, so what I suggest might not be possible (I'm coming from a RDBMS background). Can you group the teachers by the cid key, and then inside that grouping, group again, this time by the teacher key? That way, you'd end up with {"cid": "1","teacher" : [ ...]}. — FiddlingAway
– FiddlingAway, Commented Dec 25, 2022 at 22:55
Thanks for reference, but I'm unable to achieve desired output by grouping them. — Rajesh Senapati
– Rajesh Senapati, Commented Dec 28, 2022 at 13:21
One last try, then, and then I'll give up. :) Grouping seems to be the key, combined with $push. Please take a look at the following, and let me know if it works for you: mongoplayground.net/p/oACyo7-JpbN — FiddlingAway
– FiddlingAway, Commented Dec 28, 2022 at 15:41

Mandeep · Accepted Answer · 2023-01-11 06:20:50Z

1

You need to group the documents by using cid field and then push teacher document into array.

db.course.aggregate([{
    $match: {
      cid: "1"
    }
  },
  {
    $lookup: {
      localField: "teacher_id",
      from: "teacher",
      foreignField: "teacher_id",
      as: "teacher"
    }
  },
  {
    $unwind: {
      path: "$teacher",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $group: {
      _id: "$cid",
      advanced: {
        $first: "$advanced"
      },
      teacher: {
        $push: "$teacher"
      }
    }
  },
  {
    $project: {
      _id: 0,
      cid: "$_id",
      advanced: 1,
      teacher: 1
    }
  }
])

If you have any issue,you can ask

answered Jan 11, 2023 at 6:20

Mandeep

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