How to use aggregate to group by half hour with rounding? - mongodb

Question

I'm using $group to group my post by hour like:
"$group" : {
"_id" : {
"$hour" : {
$add : ["$createdAt", 10*60*60*1000]
}
},
...
}
But now I also want to group by half-of-hour, it's mean:
2:30 => 3:00
2:29 => 2:00
How I using mongo aggregate to pass this trouble?
Sr for my bad English. :)

Answer 1

I gather the +10 here is for a timezone adjustment. The same basic principles apply to producing the date with 30 minute rounding, except you want to first just convert to a numeric value and work back the intervals via a modulo ( $mod ):
{ "$group": {
"_id": {
"$add": [
{ "$subtract": [
{ "$subtract": [
{ "$add": [ "$createdAt", 1000 * 60 * 60 * 10 ] },
new Date(0)
]},
{ "$mod": [
{ "$subtract": [
{ "$add": [ "$createdAt", 1000 * 60 * 60 * 10 ] },
new Date(0)
]},
1000 * 60 * 30
]}
]},
new Date(0)
]
},
"count": { "$sum": 1 } // or whatever accumulation required
}}
Using the epoch date ( Date(0) ) with a $subtract operation from the stored date ( adjusted ) will return the milliseconds since epoch as a numeric value from the date stored. The modulo operation to the milliseconds in 30 minutes returns the remainder from the current date and you then $subtract that again to get a rounded interval.
The same is present with the $add operation where the epoch date object to a numeric value returns a Date again.
So every interval start is now the grouping key, as of every 30 minutes.
You can alternately use date aggregation operators, but this returns a BSON Date object which will be translated in API rather than just an numeric value for the "minutes" interval.
It's just standard "date math", so all the same operations apply.