GroupBy date + mongodb - node.js

Question

I have to aggregate the result based on the month of the given document. Consider the following as my document:
{
"_id" : ObjectId("5b3314a12b05b1b247366f48"),
"email" : "abc#gmail.com",
"qwerty":[{
"id" : "5ba4ebbad1b5eaf038841302",
"status" : "inprogress",
"Date" : "2018-08-20"
},
{
"id" : "5ba4ebbad1b5eaf038841303",
"status" : "inprogress",
"Date" : "2018-08-20"
}]
Following is my query:
var query =[
{ $match: {"email":email} },
{$unwind: "$courses" },
{$group:{_id:{$substrCP: ["$qwerty.Date", 5, 2]},count:{$sum:1}}}
];
Its working properly. But i $substrCP: ["$qwerty.Date", 5, 2] is based on the date format is "2018-08-20", what if "20-08-2018"?? So its possible to change the above query to accomodate of nay type.
Also i tried with new Date("").getMonth() but its showing as "NaN", i get to know that its not possible to use inside group.
Please suggest your ideas.

Answer 1

You can utilize $month in combination with $dateFromString to get what you need:
db.collection.aggregate([
{
$match: {
"email": "abc#gmail.com"
}
},
{
$unwind: "$qwerty"
},
{
$group: {
_id: {
$month: {
$dateFromString: {
dateString: "$qwerty.Date"
}
}
},
count: {
$sum: 1
}
}
}
])
You can see it here with the two different date formats.
To group per the date you can do the same without the $month:
db.collection.aggregate([
{
$match: {
"email": "abc#gmail.com"
}
},
{
$unwind: "$qwerty"
},
{
$group: {
_id: {
$dateFromString: {
dateString: "$qwerty.Date"
}
},
count: {
$sum: 1
}
}
}
])
See this version here