mongodb mapreduce group by

mongodb_mapreduce_group_by.js

db.messenger_rooms.mapReduce(
    function(){
        var date = new Date(this.created_at);
        date.setHours(date.getHours()); 
        var dateKey = (date.getFullYear()+"-"+(date.getMonth()+1)+"-"+date.getDate());
        var reserved_count = (this.reservation_status == undefined || this.reservation_status == null) ? 0 : 1;
        emit(dateKey, {count: 1, reserved_count: reserved_count});
    },
    function(key, values){
        var sum = {count: 0, reserved_count: 0}
        values.forEach(function(value){
            sum["count"] += value["count"];
            sum["reserved_count"] += value["reserved_count"];
        });
        return sum;
    },
    {
        query: {
            created_at: {$gte: ISODate("2014-04-01"), $lte: ISODate("2014-04-03")}
        },
        out: "daily_rooms_count"
    }
)