您不能在汇总管道中包含任意JavaScript，因此您正在存储 bookingdatetime 作为字符串而不是日期您不能使用

但是，由于您的日期字符串遵循严格的格式，您可以使用 $ substr 运算符从字符串中提取月份值：

  db.test.aggregate（[
 {$ group：{
 _id：{$ substr：['$ bookingdatetime'，5，2]}，
 numberofbookings：{$ sum：1 
}} 
]）
 

输出：

  {
result：[
 {
 _id：03，
numberofbookings：1 
}，
 {
_id：07，
numberofbookings b $ b}，
 {
_id：10，
numberofbookings：1 
} 
]，
ok 1 
} 
 

I am writing a query in mongo db. I have a booking table and I want to get number of bookings in a month i.e. group by month. I am confused that how to get month from a date.

Here is my schema:

{
    "_id" : ObjectId("5485dd6af4708669af35ffe6"),
    "bookingid" : 1,
    "operatorid" : 1,
    "clientid" : null,
    "callername" : "Sayem Hussain",
    "contactno" : "0205661862",
    "regular" : 0,
    "bookingdetail" : [
        {
            "driverid" : 1,
            "pickupdatetime" : "2011-04-14 11:00:00",
            "from" : "white chapel",
            "to" : "beach",
            "cost" : 500,
            "journeytime" : 60
        },
        {
            "driverid" : 2,
            "pickupdatetime" : "2012-05-14 12:00:00",
            "from" : "walthamstow",
            "to" : "mile end",
            "cost" : 1000,
            "journeytime" : 50
        }
    ],
    "bookingdatetime" : "2012-10-11T07:00:00Z"
}
{
    "_id" : ObjectId("5485dd6af4708669af35ffe7"),
    "bookingid" : 2,
    "operatorid" : 1,
    "clientid" : 1,
    "callername" : null,
    "contactno" : "0205561281",
    "regular" : 1,
    "bookingdetail" : [
        {
            "driverid" : 3,
            "pickupdatetime" : "2012-02-12 09:00:00",
            "from" : "grange park",
            "to" : "queen mary",
            "cost" : 650,
            "journeytime" : 90
        },
        {
            "driverid" : 2,
            "pickupdatetime" : "2012-02-13 06:00:00",
            "from" : "drapers bar",
            "to" : "naveed restaurant",
            "cost" : 1350,
            "journeytime" : 120
        }
    ],
    "bookingdatetime" : "2014-07-26T05:00:00Z"
}
{
    "_id" : ObjectId("5485dd6af4708669af35ffe8"),
    "bookingid" : 3,
    "operatorid" : 2,
    "clientid" : 2,
    "callername" : null,
    "contactno" : "02565138632",
    "regular" : 1,
    "bookingdetail" : [
        {
            "driverid" : 2,
            "pickupdatetime" : "2013-11-23 06:00:00",
            "from" : "hussainabad",
            "to" : "lyari",
            "cost" : 2450,
            "journeytime" : 240
        },
        {
            "driverid" : 1,
            "pickupdatetime" : "2013-11-25 08:00:00",
            "from" : "garden",
            "to" : "defence",
            "cost" : 1800,
            "journeytime" : 30
        }
    ],
    "bookingdatetime" : "2014-03-17T11:00:00Z"
}

And this is I have tried:

db.booking.aggregate([{$group:{_id:new Date("$bookingdatetime").getMonth(), numberofbookings:{$sum:1}}}])

and it returns:

{ "_id" : NaN, "numberofbookings" : 3 }

Where am I going wrong ? Kindly assist me.

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
}