MongoDB - convert string to timestamp, group by hour - mongodb

Question

I have the following documents stored in a collection:
{
"REQUESTTIMESTAMP" : "26-JUN-19 01.34.10.095000000 AM",
"UNHANDLED_INTENT" : 0,
"USERID" : "John",
"START_OF_INTENT_SKILL_CONVERSATION" : 0,
"PROPERTYCODE" : ""
}
I want to group this by the hour(which we will get from 'REQUESTTIMESTAMP')
Earlier, I had this document stored in the collection in a different way, where I had a separate field for hours, and used that hours field to group:
Previous aggregation query :
collection.aggregate([
{'$match': query}, {
'$group': {
"_id": {
"hour": "$hour",
"sessionId": "$sessionId"
}
}
}, {
"$group": {
"_id": "$_id.hour",
"count": {
"$sum": 1
}
}
}
])
Previous collection structure:
{
"timestamp" : "1581533210921",
"date" : "12-02-2020",
"hour" : "13",
"month" : "02",
"time" : "13:46:50",
"weekDay" : "Wednesday",
"__v" : 0
}
How can I do the above same Previous aggregation query with the new document structure (After extracting hours from 'REQUESTTIMESTAMP' field?)

Answer 1

You should convert your timestamp to Date object then take hour from your date object.
db.collection.aggregate([{
'$match': query
}, {
$project: {
date: {
$dateFromString: {
dateString: '$REQUESTTIMESTAMP',
format: "%m-%d-%Y" //This should be your date format
}
}
}
}, {
$group: {
_id: {
hour: {
$hour: "$date"
}
}
}
}])

Answer 2

Problem is months names are not supported by MongoDB. Either you write a lot of code or you use libraries like moments.js. First update your REQUESTTIMESTAMP to proper Date object, then you can group it.
db.collection.find().forEach(function (doc) {
var d = moment(doc.REQUESTTIMESTAMP, "DD-MMM-YY hh.mm.ss.SSS a");
db.collection.updateOne(
{ _id: doc._id },
{ $set: { date: d.toDate() } }
);
})
db.collection.aggregate([
{
$group: {
_id: { $hour: "$date" },
count: { $sum: 1 }
}
}
])

Answer 3

In case if you're not able to update DB with actual date field & still wanted to proceed with existing format, try this query it will add hour field extracted from given string field REQUESTTIMESTAMP :
Query :
db.collection.aggregate([
{
$addFields: {
hour: {
$let: {
/** split string into three parts date + hours + AM/PM */
vars: { hour: { $slice: [{ $split: ["$REQUESTTIMESTAMP", " "] }, 1, 2] } },
in: {
$cond: [{ $in: ["AM", "$$hour"] }, // Check AM exists in array
{ $toInt: { $substr: [{ $arrayElemAt: ["$$hour", 0] }, 0, 2] } }, // If yes then return int of first 2 letters of first element in hour array
{ $add: [{ $toInt: { $substr: [{ $arrayElemAt: ["$$hour", 0] }, 0, 2] } }, 12] } ] // If PM add 12 to int of first 2 letters of first element in hour array
}
}
}
}
}
])
Test : MongoDB-Playground