Convert month from number to string question in Mongodb query - mongodb

Question

I am trying to get some avg number per month in the financial year. The collection is called test and the month data comes from CreateDate field. I want to get the avg price per month. The collection data is like below:
{
"_id" : ObjectId("5fd289a93f7cf02c36837ca7"),
"ClientName" : "John",
"OrderNumber" : "12345A",
"Price" : 10,
"CreateDate" : ISODate("2020-09-20T06:00:00.000Z"),
}
{
"_id" : ObjectId("5fd289a93f7cf02c36837cc7"),
"ClientName" : "John",
"OrderNumber" : "12345",
"Price" : 20,
"CreateDate" : ISODate("2020-09-12T06:00:00.000Z"),
}
So I am writing the query to get the avg number per month by the following within the financial year (from Sep to Aug):
db.test.aggregate([
{
$match: {
"CreateDate": {
$lt: ISODate("2021-08-31T00:00:00.000Z"),
$gte: ISODate("2020-09-01T00:00:00.000Z")
}
}
},
{
$group: {
_id: {$month: "$CreateDate"},
"AvgPrice": {
"$avg": "$Price",
}
}
},
{ $project:{ _id : 0 , Month: '$_id' , "AvgPrice ": '$AvgPrice' } }
])
The result I am getting is with the following format:
{
"Month" : 9,
"AvgPrice " : 15.0
}
{
"Month" : 10,
"AvgPrice " : 18.6666666666667
}
How can I display of the month converting to a string instead of the number. For example, the following is the ideal return:
{
"Month" : Sep,
"AvgPrice" : 15.0
}
{
"Month" : Oct,
"AvgPrice" : 18.6666666666667
}
I also have two more questions:
I am using the Mongodb 3.6 version, is there any way to round up the avg price to two digit after the decimal point? For example, above will be "18.67" instead of "18.66666". Mongo 4.2 has something called $round but 3.6 seems doesn't have this function.
If I want to break down by client, has the returning result like below:
{
"ClientName": "John",
"Month" : Sep,
"AvgPrice" : 15.0
}
{
"ClientName" : "Mary"
"Month" : Oct,
"AvgPrice" : 18.6666666666667
}
How do I put another level of the group to breakdown to the client level and then month level?
Any help will be appreciated!

Answer 1

If I want to break down by client
You can add ClientName field in _id,
{
$group: {
_id: {
ClientName: "$ClientName",
month: { $month: "$CreateDate" }
},
AvgPrice: { $avg: "$Price" }
}
},
How can I display of the month converting to a string instead of the number.
There is no any straight way to get month name in mongodb, but if you prepare array of months in string and access it by index,
$arrayElemAt to select month by its number
{
$project: {
_id: 0,
ClientName: "$_id.ClientName",
Month: {
$arrayElemAt: [
["","Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],
"$_id.month"
]
},
AvgPrice: 1
}
}
Playground
I am using the Mongodb 3.6 version, is there any way to round up the avg price to two digit after the decimal point?
There is no any option in mongodb 3.6 or below, you already know there is a option $round in mongodb 4.2.
You can refer this question Rounding to 2 decimal places using MongoDB aggregation framework
, there are many tricks.