How do I convert my mongoose find query to mongoDB aggregation framework?

Question

I want to convert my existing code to mongoDB aggregation framework. Here's my code that I want to convert in aggregation.

        const comments = await Comments.find({post:req.params.id})
            .populate({
                path:"comments",
                populate:{
                    path:"author",
                    select:"name photo"
                },
                select:{
                    createdAt:1,
                    author:1,
                    _id:1,
                    body:1,
                    likes:1
                }
            })
            .populate("author","photo name")
            .select({
                createdAt:1,
                author:1,
                _id:1,
                body:1,
                comments:1,
                likes:1
            })
            .sort("-createdAt");

Answer 1

You can use below aggregation

Comments.aggregate([
  { "$match": { "post": mongoose.Types.ObjectId(req.params.id) } },
  { "$sort": { "createdAt": -1 } },
  {
    "$lookup": {
      "from": Comment.collection.name,
      "let": { "comments": "$comments" },
      "pipeline": [
        { "$match": { "$expr": { "$in": ["$_id", "$$comments"] } } },
        {
          "$lookup": {
            "from": Author.collection.name,
            "let": { "author": "$author" },
            "pipeline": [{ "$match": { "$expr": { "$eq": ["$_id", "$$author"] } } }, { "$project": { "name": 1, "photo": 1 } }],
            "as": "author"
          }
        },
        { "$unwind": "$author" },
        { "$project": { "createdAt": 1, "author": 1, "_id": 1, "body": 1, "likes": 1 } }
      ],
      "as": "comments"
    }
  },
  {
    "$lookup": {
      "from": Author.collection.name,
      "let": { "author": "$author" },
      "pipeline": [{ "$match": { "$expr": { "$eq": ["$_id", "$$author"] } } }, { "$project": { "name": 1, "photo": 1 } }],
      "as": "author"
    }
  },
  { "$unwind": "$author" },
  {
    "$project": {
      "createdAt": 1,
      "author": 1,
      "body": 1,
      "comments": 1,
      "likes": 1
    }
  }
])