How do I get comments count while fetching posts

Question

I have two collections Posts an comments. I am storing comments with postID. I want to show comments count field when fetching all the posts data. How do I achieve this?

    // posts
    {
        postID: '123',
        title: 'abc'
    }

    // comments 
    {
         postID: '123',
         comments: [
            {
                commentID: 'comment123',
                comment: 'my Comment'
            }
         ]
    }

    // Looking for this
    {
        postID: '123',
        title: 'abc',
        commentCount: 1
    }

Answer 1

Try This.

pipeline = [{
    "$lookup": {
        "from": "comments",
        "let": {
            "postId": "$postId",
        },
        "pipeline": [
            {
                "$match": {
                    "$expr": {
                        "$eq": ["$postId", "$$postId"]
                    },
                }
            },
            {
                "$group": {
                    "_id": "$postId",
                    "comments_count": {"$sum": 1}
                }
            }
        ],
        "as": "comments"
    }
},
{
    "$project": {
        "_id": 0,
        "postId": 1,
        "title":1,
        "comments_count": "$comments.comments_count"
    }
}]

db.posts.aggregate(pipeline)

Answer 2

Here's one way you could do it.

db.posts.aggregate([
  {
    "$lookup": {
      "from": "comments",
      "localField": "postID",
      "foreignField": "postID",
      "pipeline": [
        {
          "$project": {
            "_id": 0,
            "commentCount": {"$size": "$comments"}
          }
        }
      ],
      "as": "commentCount"
    }
  },
  {
    "$project": {
      "_id": 0,
      "postID": 1,
      "title": 1,
      "commentCount": {"$first": "$commentCount.commentCount"}
    }
  }
])

Try it on mongoplayground.net.