db.vehicle_alerts.aggregate([
    { "$sort": { "created_at": 1 } },
    { "$group": {
        "_id": {
            "type":"$category_type",
            "time":{ "$add": [
                { "$subtract": [
                    { "$subtract": [ "$created_at", new Date(0) ] },
                    { "$mod": [
                        { "$subtract": [ "$created_at", new Date(0) ] },
                        1000 * 60 * 5
                    ]}
                ]},
                new Date(0)
            ]}
        },
        "count":{
            "$sum": 1
        }
    }}
]);