Can I group floating point numbers by range in MongoDB? - mongodb

Question

I have a MongoDB set up with documents like this
{
"_id" : ObjectId("544ced7b9f40841ab8afec4e"),
"Measurement" : {
"Co2" : 38,
"Humidity" : 90
},
"City" : "Antwerp",
"Datum" : ISODate("2014-10-01T23:13:00.000Z"),
"BikeId" : 26,
"GPS" : {
"Latitude" : 51.20711593206187,
"Longitude" : 4.424424413969158
}
}
Now I try to aggregate them by date and location and also add the average of the measurement to the result. So far my code looks like this:
db.stadsfietsen.aggregate([
{$match: {"Measurement.Co2": {$gt: 0}}},
{
$group: {
_id: {
hour: {$hour: "$Datum"},
Location: {
Long: "$GPS.Longitude",
Lat: "$GPS.Latitude"
}
},
Average: {$avg: "$Measurement.Co2"}
}
},
{$sort: {"_id": 1}},
{$out: "Co2"}
]);
which gives me a nice list of all the possible combinations of hour and GPS coordinates, in this form:
{
"_id" : {
"hour" : 0,
"Location" : {
"Long" : 3.424424413969158,
"Lat" : 51.20711593206187
}
},
"Average" : 82
}
The problem is that there are so many unique coordinates, that it's not useful.
Can I group the documents together when there are values that are close together? Say from Longitude 51.207 to Longitude 51.209?

Answer 1

There is no standard support for ranges in $group.
Mathematically
You could calculate a new value that will be the same for several geolocations. For example you could simulate a floor method:
_id:{ hour:{$hour:"$Datum"}, Location:{
Long: $GPS.Longitude - mod($GPS.Longitude, 0.01),
Lat: $GPS.Latitude - mod($GPS.Latitude, 0.01)
}}
Geospatial Indexing
You could restructure you're application to use a Geospatial index and search for all locations in a given range. If this is applicable depends very much on your use case.
Map-Reduce
Map-Reduce is more powerful than the aggregation framework. You can definitely use this to do your calculations, but it's more complex and therefore I can't present you a ready-made solution without spending another hour.