MongoDB MapReduce: Not working as expected for more than 1000 records - mongodb

Question

I wrote a mapreduce function where the records are emitted in the following format
{userid:<xyz>, {event:adduser, count:1}}
{userid:<xyz>, {event:login, count:1}}
{userid:<xyz>, {event:login, count:1}}
{userid:<abc>, {event:adduser, count:1}}
where userid is the key and the remaining are the value for that key.
After the MapReduce function, I want to get the result in following format
{userid:<xyz>,{events: [{adduser:1},{login:2}], allEventCount:3}}
To acheive this I wrote the following reduce function
I know this can be achieved by group by.. both in aggregation framework and mapreduce, but we require a similar functionality for a complex scenario. So, I am taking this approach.
var reducefn = function(key,values){
var result = {allEventCount:0, events:[]};
values.forEach(function(value){
var notfound=true;
for(var n = 0; n < result.events.length; n++){
eventObj = result.events[n];
for(ev in eventObj){
if(ev==value.event){
result.events[n][ev] += value.allEventCount;
notfound=false;
break;
}
}
}
if(notfound==true){
var newEvent={}
newEvent[value.event]=1;
result.events.push(newEvent);
}
result.allEventCount += value.allEventCount;
});
return result;
}
This runs perfectly, when I run for 1000 records, when there are 3k or 10k records, the result I get is something like this
{ "_id" : {...}, "value" :{"allEventCount" :30, "events" :[ { "undefined" : 1},
{"adduser" : 1 }, {"remove" : 3 }, {"training" : 1 }, {"adminlogin" : 1 },
{"downgrade" : 2 } ]} }
Not able to understand where this undefined came from and also the sum of the individual events is less than allEventCount. All the docs in the collection has non-empty field event so there is no chance of undefined.
Mongo DB version -- 2.2.1
Environment -- Local machine, no sharding.
In the reduce function, why should this operation fail result.events[n][ev] += value.allEventCount; when the similar operation result.allEventCount += value.allEventCount; passes?
The corrected answer as suggested by johnyHK
Reduce function:
var reducefn = function(key,values){
var result = {totEvents:0, event:[]};
values.forEach(function(value){
value.event.forEach(function(eventElem){
var notfound=true;
for(var n = 0; n < result.event.length; n++){
eventObj = result.event[n];
for(ev in eventObj){
for(evv in eventElem){
if(ev==evv){
result.event[n][ev] += eventElem[evv];
notfound=false;
break;
}
}}
}
if(notfound==true){
result.event.push(eventElem);
}
});
result.totEvents += value.totEvents;
});
return result;
}

Answer 1

The shape of the object you emit from your map function must be the same as the object returned from your reduce function, as the results of a reduce can get fed back into reduce when processing large numbers of docs (like in this case).
So you need to change your emit to emit docs like this:
{userid:<xyz>, {events:[{adduser: 1}], allEventCount:1}}
{userid:<xyz>, {events:[{login: 1}], allEventCount:1}}
and then update your reduce function accordingly.