mongodb aggregation cast to int - mongodb

Question

I have the following problem with mongo using the aggregation framework.
Suppose and item with time in seconds, t, and an event id occurring, e, like:
item:{t:11433, e:some_id}
what I want is to aggregate according to t and e. It means counting the number of id 'e' in a time t.
This is easy to do using the aggregation with $group.
However, I would like to have a different time course. For example, I want to count number of same event id in a time slot of eg. 5 seconds. I could do this progammatically, in js or python . I was just wondering if it could work using just mongo, using a cascade of group.
I tried to project using $divide[t,10]. For 11433, this would give, 1143.3 But it seems that I can't remove the 0.3 in Mongo (Otherwise I could group in this other scale).
Any hint?
thanks

Answer 1

To get an integer group key for a 5-second interval, you could use the formula
t = t - (t % 5) // % is the modula operator
In the aggregation framework this would look like this:
db.xx.aggregate([
// you need two projections, as they can not be nested
// this does not work:
// { $project: { _id: 0, e: 1, t: 1, tk: { $subtract: [ "$t", $mod: [ "$t", 5 ] ] } } },
//
// get modula 5 of time in seconds:
{ $project: { _id: 0, e: 1, t: 1, tm5: { $mod: [ "$t", 5 ] } } },
// subtract it from time:
{ $project: { _id: 0, e: 1, ti: { $subtract: [ "$t", "$tm5" ] } } },
// now group on e and interval,
{ $group: { _id: { e: "$e", interval: "$ti" }, count: { $sum: 1 } } },
])
For this example collection:
> db.xx.find()
{ "_id" : ObjectId("515e5a7157a0887a97cc8d1d"), "t" : 11433, "e" : "some_id" }
{ "_id" : ObjectId("515e60d457a0887a97cc8d1e"), "t" : 11434, "e" : "some_id" }
{ "_id" : ObjectId("515e60d857a0887a97cc8d1f"), "t" : 11438, "e" : "some_id" }
the result is:
{
"result" : [
{
"_id" : {
"e" : "some_id",
"interval" : 11435
},
"count" : 1
},
{
"_id" : {
"e" : "some_id",
"interval" : 11430
},
"count" : 2
}
],
"ok" : 1
}