Need some help completing this aggregation pipeline - mongodb

Question

I have an analytics collection where I store queries as individual documents. I want to count the number of queries taking place over the past day (24 hours). Here's the aggregation command as it is:
db.analytics.aggregate([{$group:{_id:{"day":{$dayOfMonth:"$datetime"},"hour":{$hour:"$datetime"}},"count":{$sum:1}}},{$sort:{"_id.day":1,"_id.hour":1}}])
The result looks like:
.
.
.
{
"_id" : {
"day" : 17,
"hour" : 19
},
"count" : 8
},
{
"_id" : {
"day" : 17,
"hour" : 22
},
"count" : 1
},
{
"_id" : {
"day" : 18,
"hour" : 0
},
"count" : 1
}
.
.
.
Originally, my plan was to add a $limit operation to simply take the last 24 results. That's a great plan until you realize that there are some hours without any queries at all. So the last 24 documents could go back more than a single day. I thought of using $match, but I'm just not sure how to go about constructing it. Any ideas?

Answer 1

First of all you need to get the day just as current date or as most recent document from the collection. Then use query for specified day like:
db.analytics.aggregate([
{$project:{datetime:"$datetime",day:{$dayOfMonth:"$datetime"}}},
{$match:{day:3}},
{$group:{_id:{"hour":{$hour:"$datetime"}},"count":{$sum:1}}},
{$sort:{"_id.hour":1}}
]);
where 3 is the day of the month here {$match:{day:3}}
The idea is to add a day field, so, we able to filter by it, then group documents of the day by hours and sort.