MongoDB MapReduce Standard Deviation over date range - mongodb

Question

Good Afternoon SO, I wonder if anybody could help me. I am currently investigating using MongoDB Aggregation Framework and MapReduce functions.
My dataset looks like this
[{
"Name" : "Person 1",
"RunningSpeed" : [{
"Date" : ISODate("2005-07-23T23:00:00.000Z"),
"Value" : 10
}, {
"Date" : ISODate("2006-07-23T23:00:00.000Z"),
"Value" : 20
}, {
"Date" : ISODate("2007-07-23T23:00:00.000Z"),
"Value" : 30
}, {
"Date" : ISODate("2008-07-23T23:00:00.000Z"),
"Value" : 40
}
]
}, {
"Name" : "Person 2",
"RunningSpeed" : [{
"Date" : ISODate("2005-07-23T23:00:00.000Z"),
"Value" : 5
}, {
"Date" : ISODate("2006-07-23T23:00:00.000Z"),
"Value" : 10
}, {
"Date" : ISODate("2007-07-23T23:00:00.000Z"),
"Value" : 20
}, {
"Date" : ISODate("2008-07-23T23:00:00.000Z"),
"Value" : 40
}
]
}, {
"Name" : "Person 3",
"RunningSpeed" : [{
"Date" : ISODate("2005-07-23T23:00:00.000Z"),
"Value" : 20
}, {
"Date" : ISODate("2006-07-23T23:00:00.000Z"),
"Value" : 10
}, {
"Date" : ISODate("2007-07-23T23:00:00.000Z"),
"Value" : 30
}, {
"Date" : ISODate("2008-07-23T23:00:00.000Z"),
"Value" : 25
}
]
}
]
I have done a lot of research and as I an see there is no out of the box support for doing SD calculations. I have reviewed a few links and SO posts and came up with this URL https://gist.github.com/RedBeard0531/1886960, which seems to be what I am looking for.
So enough about the background what I would like to do is generate a chart of SDs over each year.
The current function does not take inconsideration each year only the value as a whole. I have changed the map function to and have no idea where to put the group date function.
function map() {
emit(1, // Or put a GROUP BY key here
{sum: this.RunningSpeed.value, // the field you want stats for
min: this.RunningSpeed.value,
max: this.RunningSpeed.value,
count:1,
diff: 0, // M2,n: sum((val-mean)^2)
});
}
However I just get zero's. Could anybody help me adapt this function?

Answer 1

You need to use getFullYear() and a forEach loop through each RunningSpeed entry:
function map() {
this.RunningSpeed.forEach(function(data){
var year = data.Date.getFullYear();
emit(year, // Or put a GROUP BY key here, the group by key is Date.getFullYear()
{sum: data.Value, // the field you want stats for
min: data.Value,
max: data.Value,
count: 1,
diff: 0, // M2,n: sum((val-mean)^2)
});
});
}