Getting instance count from an inner pipeline

Working with Data
,
1

Hello, I have created an application which has a follower-followee feature. So if I follow user A and B, I will see posts from users A and B in my feed. People can comment on the post. Here is the query that I used:

const posts = await Post.aggregate([
      // match followee posts
      {
        $lookup: {
          from: 'followers',
          localField: 'author',
          foreignField: 'to',
          as: 'rel'
        }
      },
      { $match: { 'rel.from': mongoose.Types.ObjectId(userId) }},
      // Get first 2 comments
      {
        $lookup: {
          from: 'comments',
          as: 'comments',
          let: { post_id: '$_id' },
          pipeline: [
            {
              $match: {
                $expr: { $eq: ['$post', '$$post_id'] }
              }
            },
            { $sort: { createdAt: -1 } },
            { $limit: 2 }
          ]
        },
      },
      {
        $project: {
          'rel': 0
        }
      }
    ]).exec();

What it does is:

  • Match the posts according to followee
  • Attach the first 2 comments with each post

Now, I want to count total number of comments in each post and attach them with the post as totalComments . What would the efficient approach to do this? I’ve wasted a lot of time behind this but can’t find a way