MongoDB Lookup with pipelines

ex1.js

Venue.aggregate([
  { "$match": { "_id": mongoose.Types.ObjectId(id.id) } },
  { "$lookup": {
    "from": Review.collection.name,
    "let": { "reviews": "$reviews" },
    "pipeline": [
       { "$match": { "$expr": { "$in": [ "$_id", "$$reviews" ] } } },
       { "$lookup": {
         "from": Comment.collection.name,
         "let": { "comments": "$comments" },
         "pipeline": [
           { "$match": { "$expr": { "$in": [ "$_id", "$$comments" ] } } },
           { "$lookup": {
             "from": Author.collection.name,
             "let": { "author": "$author" },
             "pipeline": [
               { "$match": { "$expr": { "$eq": [ "$_id", "$$author" ] } } },
               { "$addFields": {
                 "isFollower": { 
                   "$in": [ 
                     mongoose.Types.ObjectId(req.user.id),
                     "$followers"
                   ]
                 }
               }}
             ],
             "as": "author"
           }},
           { "$addFields": { 
             "author": { "$arrayElemAt": [ "$author", 0 ] }
           }}
         ],
         "as": "comments"
       }},
       { "$sort": { "createdAt": -1 } }
     ],
     "as": "reviews"
  }},
 ])

ex2.js

Venue.aggregate([
  { "$match": { "_id": mongoose.Types.ObjectId(id.id) } },
  { "$lookup": {
    "from": Review.collection.name,
    "localField": "reviews",
    "foreignField": "_id",
    "as": "reviews"
  }},
  { "$unwind": "$reviews" },
  { "$lookup": {
    "from": Comment.collection.name,
    "localField": "reviews.comments",
    "foreignField": "_id",
    "as": "reviews.comments",
  }},
  { "$unwind": "$reviews.comments" },
  { "$lookup": {
    "from": Author.collection.name,
    "localField": "reviews.comments.author",
    "foreignField": "_id",
    "as": "reviews.comments.author"
  }},
  { "$unwind": "$reviews.comments.author" },
  { "$addFields": {
    "reviews.comments.author.isFollower": {
      "$in": [ 
        mongoose.Types.ObjectId(req.user.id), 
        "$reviews.comments.author.followers"
      ]
    }
  }},
  { "$group": {
    "_id": { 
      "_id": "$_id",
      "reviewId": "$review._id"
    },
    "name": { "$first": "$name" },
    "addedBy": { "$first": "$addedBy" },
    "review": {
      "$first": {
        "_id": "$review._id",
        "createdAt": "$review.createdAt",
        "venue": "$review.venue",
        "author": "$review.author",
        "content": "$review.content"
      }
    },
    "comments": { "$push": "$reviews.comments" }
  }},
  { "$sort": { "_id._id": 1, "review.createdAt": -1 } },
  { "$group": {
    "_id": "$_id._id",
    "name": { "$first": "$name" },
    "addedBy": { "$first": "$addedBy" },
    "reviews": {
      "$push": {
        "_id": "$review._id",
        "venue": "$review.venue",
        "author": "$review.author",
        "content": "$review.content",
        "comments": "$comments"
      }
    }
  }}
])