Map Reducing with timezones - mongodb

Question

I'm trying to map reduce a bunch of data in order to generate a daily graph, the catch is that the application has users from all over the world and they want the data in their own timezone.
The current map reduction I have is pretty simple
var map = function(){
var userLogin = this;
var d = this.StartTime;
var start = d.getFullYear() + '-' + d.getMonth() + 1, + '-' + d.getDate();
var reduceValue = {
SuccessSession: 0,
FailSession: 0
}
if(userLogin.ExitReason.Severity <2)
{
reduceValue.SuccessSession += 1;
} else {
reduceValue.FailSession += 1;
}
emit({ClientId: this.ClientId, StartDate:start, IsAdmin: this.IsAdmin},
{SuccessSession: reduceValue.SuccessSession, FailSession: reduceValue.FailSession })
}
and the Reduce
var reduce = function(key, value) {
var reducedValue = {
SuccessSession: 0,
FailSession : 0
};
for(var idx = 0; idx < value.length; idx++)
{
reducedValue.SuccessSession += value[idx].SuccessSession;
reducedValue.FailSession += value[idx].FailSession;
}
return reducedValue;
};
The problem with this approach is that it calculates from midnight UTC to midnight UTC, however I'd prefer to be able to query the Map Reduced and still be able to query depending upon which timezone a user is in.
Is there any simple way to accomplish being able to mapreduce and yet keep the ability to use timezones?

Answer 1

No easy way. Map is a javascript, so you will need at least IANA TZ database there. Something like https://momentjs.com/timezone/ being available within the map function.
Would you consider aggregation framework instead of map-reduce? It has support of timezones and is much faster. The pipeline could be as simple as this:
db.collection.aggregate([
{ $group: {
_id: {
ClientId: "$ClientId",
start: { $dateToString: {
format: "%Y-%m-%d",
date: "$StartTime",
timezone: "Europe/Kiev"
} },
IsAdmin: "$IsAdmin"
},
"SuccessSession": { $sum: { $cond: {
if: { $lt: [ "$ExitReason.Severity", 2 ] },
then: 1,
else: 0
} } },
"FailSession":{ $sum: { $cond: {
if: { $lt: [ "$ExitReason.Severity", 2 ] },
then: 0,
else: 1
} } },
} }
])