$matching with field added with $dateToString doesn't work - mongodb

Question

In my MongoDB collection I have documents that contain a nested string field, containing a month and year, e.g. '04/2021'. Sample document:
{
"_id": {
"$oid": "608ba45cec43c5b24cda034b"
},
"status": "pass",
"stage": 5,
"priority": 0,
"payload": {
"company_id": "8800",
"company_name": "<MY COMPANY>",
"target_period": "04/2021"
},
"retry_count": 0,
"build_number": "101",
"job_name": "P123",
"createdAt": {
"$date": "2021-04-30T06:31:56.000Z"
},
"updatedAt": {
"$date": "2021-05-10T03:55:44.686Z"
}
}
I am trying to write an aggregation pipeline that will dynamically return documents where said field points to the past month. For example, ran this month (May 2021) I would get documents labeled with '04/2021'. From this post I found the oneliner for getting the comparison string: new Date(new Date().getFullYear(), new Date().getMonth(), 1). (I understand that by the virtue of getMonth returning a zero-based index of month, getting the previous month works by accident and has to be solved somehow.)
This pipeline does not work:
[
{
$addFields: {
previous_month: {
$dateToString: {
'date': new Date(new Date().getFullYear(), new Date().getMonth(), 1),
'format': '%m/%G'
}
}
}
},
{
$match: {
"payload.target_period": "$previous_month"
}
}
]
With MongoDB Compass I can see that the field previous_month is populated just fine by the $addFields stage (above sample document gets value 04/2021), but the $match stage returns 0 documents. I'm running MongoDB version 4.2.12.

Answer 1

You should use $expr operator while trying to self reference another ket in a document inside
$match stage.
[
{
$addFields: {
previous_month: {
$dateToString: {
'date': new Date(new Date().getFullYear(), new Date().getMonth(), 1),
'format': '%m/%G'
}
}
}
},
{
$match: {
$expr: {
$eq: [ "$payload.target_period", "$previous_month" ],
},
}
}
]

Answer 2

Instead of doing whole process in query, I think you can prepare input date in your client language, (js, nodejs) easily,
have prepared a function zeroFill it will return number with concat 0 if its less than 10,
get previous month date and pick previous month
concat both month and year
function zeroFill(i) { return (i < 10 ? '0' : '') + i; }
var date = new Date();
date.setMonth(date.getMonth() - 1);
let searchDate = zeroFill(date.getMonth() + 1) + "/" + date.getFullYear();
console.log(searchDate); // mm/yyyy
Your query would be just:
[{ $match: { "payload.target_period": searchDate } }]
Playground

Answer 3

I would suggest moment.js library, it is much simpler to use:
{ $match: { "payload.target_period": moment().startOf("months").subtract(1, "months").format("MM/YYYY") } }