aggregator

a.js

db.website_performance.aggregate(
[
    {
        $group: {
            _id: "$domain",
            sum_impressions: {
                $sum: "$impressions"
            },
            sum_bids: {
                $sum: "$bids"
            }
        }
    },
    {
        $project: {
            ctr: {
                $multiply: [
                    100,
                    {
                        $divide: [
                            "$sum_impressions",
                            "$sum_bids"
                        ]
                    }
                ]
            }
        }
    },
    {
        $sort: {
            ctr: -1
        }
    },
    {
        $limit: 0
    }
]);