Category aggregate query

category_aggregate.js

var getTopCategory = function(category, callback) {
        var sorter = {};
        sorter[category] = -1;
        this.collection('results').aggregate([
                { $sort: sorter },
                { $project: {
                        _id: 0,
                        name: 1,
                        category_value: '$' + category
                }},
                { $limit: 1 },

        ], function(err, result) {
                callback(err, result);
        });
}

promise.js

var topCategoryList = function(db, callback) {
        var map = ['kills', 'damage', 'round', 'waveClears'],
                list = {};
        if (!db) {
                callback('DB not available.');
                return;
        }

        Async.map(map, _.bind(getTopCategory, db), function(err, results) {
                if (err) {
                        callback(err);
                        return;
                }
                _.each(results, function(result, index) {
                        list[map[index]] = result[0];
                })
                callback(null, list);
        });
}