Mongo DB query to return count?

Question

I have two collections Course and File . This is my data from Course collection

[{
    _id: 1,
    name: "Course1",
    price: "100"
},
{
    _id: 2,
    name: "Course2",
    price: "200"
}]

This is my data from files collection

[{
    _id: 1,
    name: "File1",
    isFree: true,
    courseId: 1
}
{
    _id: 2,
    name: "File2",
    isFree: false,
    courseId: 1
}
{
    _id: 3,
    name: "File3",
    isFree: false,
    courseId: 1
}]

I want to retrieve Course 1 with Course name , Count of files which are free , Count of files which are not free .

My attempt:

Course.aggregate([
    {$lookup:{
        from:"files",
        localField: "_id",
        foreignField: "courseId",
        as: "files"
    }},
])

This is my response If I execute my above query:

[
    {
        "_id": "6161bdcc0aada09d81598840",
        "name": "Course1",
        "files": [
            {
                "_id": "6166a0814ad3f56b71232b9c",
                "name": "File1.jpeg",
                "isFree":true
            },
            {
                "_id": "6166a0814ad3f56b71232b9d",
                "name": "File2.jpeg",
                "isFree":false
            },
            {
                "_id": "6166a0814ad3f56b71232b9v",
                "name": "File3.jpeg",
                "isFree":false
            }
        ]
    }
]

I want my output as:

[
    {
        "_id": "6161bdcc0aada09d81598840",
        "name": "Course1",
        "files": {"free": 1,"paid": 2}
    }
]

Answer 1

You can try lookup with pipeline,

• $lookup with pipeline, pass courseId in let
• $match courseId condition
• $group by isFeee and get total count
• $project to show required fields in k(key) and v(value) format
• $cond to check if isFree true then return "free" otherwise return "paid"
• $addFields to update files array
• $arrayToObject convert files key-value array to an object
• $ifNull to check if property is not exists then set 0

await Course.aggregate([
  {
    $lookup: {
      from: "files",
      let: { courseId: "$_id" },
      pipeline: [
        { $match: { $expr: { $eq: ["$$courseId", "$courseId"] } } },
        {
          $group: {
            _id: "$isFree",
            count: { $sum: 1 }
          }
        },
        {
          $project: {
            _id: 0,
            k: { $cond: ["$_id", "free", "paid"] },
            v: "$count"
          }
        }
      ],
      as: "files"
    }
  },
  {
    $addFields: {
      files: { $arrayToObject: "$files" }
    }
  },
  {
    $addFields: {
      "files.free": { $ifNull: ["$files.free", 0] },
      "files.paid": { $ifNull: ["$files.paid", 0] }
    }
  }
])

Playground

Answer 2

Try following:

db.course.aggregate([
  {
    $lookup: {
      from: "files",
      localField: "_id",
      foreignField: "courseId",
      as: "files",
    },
  },
  {
    $unwind: {
      path: "$files",
      preserveNullAndEmptyArrays: true,
    },
  },
  {
    $group: {
      _id: "$_id",
      name: {
        $first: "$name",
      },
      price: {
        $first: "$price",
      },
      free: {
        $sum: {
          $cond: [
            {
              $eq: ["$files.isFree", true],
            },
            1,
            0,
          ],
        },
      },
      paid: {
        $sum: {
          $cond: [
            {
              $eq: ["$files.isFree", false],
            },
            1,
            0,
          ],
        },
      },
    },
  },
  {
    $addFields: {
      "files.free": "$free",
      "files.paid": "$paid",
    },
  },
  {
    $project: {
      _id: 1,
      name: 1,
      files: 1,
      price: 1,
    },
  },
]);