Get last inserted item per day - mongodb

Question

Is there a feature in mongodb that I can use to get the last inserted item per day ? I have a collection where I need to get the last inserted item per day, the data is grouped on an hourly basis like in the structure below.
{
timestamp: 2017-05-04T09:00:00.000+0000,
data: {}
},
{
timestamp: 2017-05-04T10:00:00.000+0000,
data: {}
}
I thought about using a projection but I am not quite sure how I could do this.
Edit: Also, since mongodb stores data in UTC, I would like to account for the offset as well.

Answer 1

You can $sort and use $last for the item, with rounding out the grouping key to each day:
db.collection.aggregate([
{ "$sort": { "timestamp": 1 } },
{ "$group": {
"_id": {
"$add": [
{ "$subtract": [
{ "$subtract": [ "$timestamp", new Date(0) ] },
{ "$mod": [
{ "$subtract": [ "$timestamp", new Date(0) ] },
1000 * 60 * 60 * 24
]}
]},
new Date(0)
]
},
"lastDoc": { "$last": "$$ROOT" }
}}
])
So the sort makes things appear in order, and then the grouping _id is rounded for each day by some date math. You subtract the epoch date from the current date to make it a number. Use the modulus to round to a day, then add the epoch date to the number to return a Date.
So stepping through the math we have getting the timestamp value from the date with the $subract line. We do this a couple of times:
{ "$subtract": [ "$timestamp", new Date(0) ] }
// Is roughly internally like
ISODate("2017-06-06T10:44:37.627Z") - ISODate("1970-01-01T00:00:00Z")
1496745877627
Then there is the modulo with $mod which when applied to the numeric value returns the difference. The 1000 milliseconds * 60 seconds * 60 * minutes * 24 hours gives the other argument:
{ "$mod": [
{ "$subtract": [ "$timestamp", new Date(0) ] },
1000 * 60 * 60 * 24
]}
// Equivalent to
1496745877627 % (1000 * 60 * 60 * 24)
38677627
Then there is the wrapping $subtract of the two numbers:
{ "$subtract": [
{ "$subtract": [ "$timestamp", new Date(0) ] },
{ "$mod": [
{ "$subtract": [ "$timestamp", new Date(0) ] },
1000 * 60 * 60 * 24
]}
]}
// Subtract "difference" of the modulo to a day
// from the milliseconds value of the current date
1496745877627 - 38677627
1496707200000
Then add back to the epoch date value to create a date rounded to the current day, which to the aggregation pipeline basically looks like providing the millisecond value to the constructor:
new Date(1496707200000)
ISODate("2017-06-06T00:00:00Z")
Which takes the timestamp value and subrtacts out the difference of the divisor from "one day" and ends up at the time at the "start of day".
Just using $$ROOT here to represent the whole document. But any document path provided to $last here provides the result.