Group and count by month - mongodb

Question

I have a booking table and I want to get number of bookings in a month i.e. group by month.
And I am confused that how to get month from a date.
Here is my schema:
{
"_id" : ObjectId("5485dd6af4708669af35ffe6"),
"bookingid" : 1,
"operatorid" : 1,
...,
"bookingdatetime" : "2012-10-11T07:00:00Z"
}
{
"_id" : ObjectId("5485dd6af4708669af35ffe7"),
"bookingid" : 2,
"operatorid" : 1,
...,
"bookingdatetime" : "2014-07-26T05:00:00Z"
}
{
"_id" : ObjectId("5485dd6af4708669af35ffe8"),
"bookingid" : 3,
"operatorid" : 2,
...,
"bookingdatetime" : "2014-03-17T11:00:00Z"
}
And this is I have tried:
db.booking.aggregate([
{ $group: {
_id: new Date("$bookingdatetime").getMonth(),
numberofbookings: { $sum: 1 }
}}
])
but it returns:
{ "_id" : NaN, "numberofbookings" : 3 }
Where am I going wrong?

Answer 1

You need to use the $month keyword in your group. Your new Date().getMonth() call will only happen once, and will try and create a month out of the string "$bookingdatetime".
db.booking.aggregate([
{$group: {
_id: {$month: "$bookingdatetime"},
numberofbookings: {$sum: 1}
}}
]);

Answer 2

You can't include arbitrary JavaScript in your aggregation pipeline, so because you're storing bookingdatetime as a string instead of a Date you can't use the $month operator.
However, because your date strings follow a strict format, you can use the $substr operator to extract the month value from the string:
db.test.aggregate([
{$group: {
_id: {$substr: ['$bookingdatetime', 5, 2]},
numberofbookings: {$sum: 1}
}}
])
Outputs:
{
"result" : [
{
"_id" : "03",
"numberofbookings" : 1
},
{
"_id" : "07",
"numberofbookings" : 1
},
{
"_id" : "10",
"numberofbookings" : 1
}
],
"ok" : 1
}

Answer 3

Starting in Mongo 4, you can use the $toDate operator to convert your string to date (building on the answer given by Will Shaver):
// { date: "2012-10-11T07:00:00Z" }
// { date: "2012-10-23T18:30:00Z" }
// { date: "2012-11-02T21:30:00Z" }
db.bookings.aggregate([
{ $group: {
_id: { month: { $month: { $toDate: "$date" } } },
bookings: { $sum: 1 }
}}
])
// { "_id" : { "month" : 10 }, "bookings" : 2 }
// { "_id" : { "month" : 11 }, "bookings" : 1 }

Answer 4

If you intend to get groups by months even if your data spreads over multiple years, you can use a combination of $dateFromString and $dateToString (in order to format dates as "%Y-%m" (e.g. 2012-10)):
// { date: "2012-10-11T07:00:00Z" }
// { date: "2012-10-23T18:30:00Z" }
// { date: "2012-11-02T21:30:00Z" }
// { date: "2013-01-11T18:30:00Z" }
// { date: "2013-10-07T14:15:00Z" }
db.bookings.aggregate([
{ $group: {
_id: {
$dateToString: {
date: { $dateFromString: { dateString: "$date" } },
format: "%Y-%m"
}
},
bookings: { $count: {} } // or { $sum: 1 } prior to Mongo 5
}}
])
// { _id: "2012-10", bookings: 2 }
// { _id: "2012-11", bookings: 1 }
// { _id: "2013-01", bookings: 1 }
// { _id: "2013-10", bookings: 1 }
This:
first transforms the string date into a string: $dateFromString: { dateString: "$date" }
in order to format the date as %Y-%m: $dateToString: { date: { }, format: "%Y-%m" }
the combination of which ($dateFromString/$dateToString) is used as our group key
and finally we count our grouped bookings with $count (or { $sum: 1 } prior to Mongo 5)