MongoDB - Comment Upvoting/Downvoting with Aggregation Pipeline

Question

I\'m trying to implement an upvote/downvote mechanism for comments (similar to the upvoting/downvoting mechanism found on reddit). I have a separate collection called comm

Answer 1

you can do it with the following pipeline update but it requires that the upvotes and downvotes arrays exist. even if it's just empty.

var comment_id = ObjectId("5e5983102328a83d1a4b541f");
var user_id = ObjectId("5e5983102328a83d1a4b53e5");

db.commentReputation.update(
    {
        commentId: comment_id
    },
    [
        {
            $set: {
                upvotes: {
                    $cond: [
                        { $in: [user_id, '$upvotes'] },
                        { $setDifference: ['$upvotes', [user_id]] },
                        { $setUnion: ['$upvotes', [user_id]] }
                    ]
                }
            }
        },
        {
            $set: {
                downvotes: {
                    $cond: [
                        { $in: [user_id, '$downvotes'] },
                        { $setDifference: ['$downvotes', [user_id]] },
                        '$downvotes'
                    ]
                }
            }
        }
    ]
);