For example, I have two collections, post and comment.

The query code is

post.aggregate([
    {
        $lookup: {
            from: 'comment',
            localField: '_id',
            foreignField: 'postId',
            as: 'post_comment'
        },
    }
])

The result is like

{
  _id: new ObjectId('665fa2e22ed07dbfa654cef1'),
  post_comment: [
    {
      _id: new ObjectId('665fb02cf3176331944fbbee'),
      postId: new ObjectId('665fa2e22ed07dbfa654cef1'),
      text: 'comment 1'
    }
  ]
}

But I don’t need the details of the comments, I only need the number of comments of each post.

The desired result is like

{
  _id: new ObjectId('665fa2e22ed07dbfa654cef1'),
  post_comment: [
    {
        count: 1          // newly changed.
    }
  ]
}

Or simply

{
  _id: new ObjectId('665fa2e22ed07dbfa654cef1'),
  comment_count: 1        // newly changed.
}

Could anyone teach me how to do it?

Thanks!