Mongo query Update year depends on the inner document field - arrays

Question

Here is my data. I wanted to change year but It should effective to only the first item of the document array
{
"_id": {
"$oid": "62053aa8aa1cfbe8c4e72662"
},
"school": "Test",
"reports": [
{
"year": "2020", // This has to be changed to 2019
"createdAt": {
"$date": "2022-02-10T17:05:25.682Z"
},
"pid": {
"$oid": "620545d5097761628f32365a"
},
"details": {
"end_date": {
"$date": "2020-03-31T00:00:00.000Z" // when end date is prior to July 01 of the $year mentioned.
}
}
}, {
"year": "2020",
"createdAt": {
"$date": "2022-03-14T19:08:38.125Z"
},
"pid": {
"$oid": "622f92b68a408531d4b784de"
},
"details": {
"end_date": {
"$date": "2021-03-31T00:00:00.000Z"
}
}
}
]
}
In the above data, I want to reduce the year to the previous year, if details.end_date is prior to July 01 of the current mentioned year. But It should change only the first item of the embedded array.
For example,
If Year is 2020 and details.end_date is prior to 01-July-2020, then change the year to 2019
If Year is 2020 and details.end_date is after 01-July-2020, then do not change the year
If Year is 2021 and details.end_date is prior to 01-July-2021, then change the year to 2020
If Year is 2021 and details.end_date is after 01-July-2021, then do not change the year

Answer 1

You can do the followings in an aggregation pipeline:
isolate the first elem of the reports array for easier processing using $arrayElemAt
use $cond to derive the year value with $month. Use $toInt and $toString for type conversion
use $concatArrays to append back the processed first Elem back to the reports array. Keep only the "tail" (i.e. without the first elem) using $slice
$merge to update the result back to the collection
db.collection.aggregate([
{
"$addFields": {
"firstElem": {
"$arrayElemAt": [
"$reports",
0
]
}
}
},
{
"$addFields": {
"firstElem.year": {
"$cond": {
"if": {
$lt: [
{
"$month": "$firstElem.end_date"
},
7
]
},
"then": {
"$toString": {
"$subtract": [
{
"$toInt": "$firstElem.year"
},
1
]
}
},
"else": "$firstElem.year"
}
}
}
},
{
"$addFields": {
"reports": {
"$concatArrays": [
[
"$firstElem"
],
{
"$slice": [
"$reports",
1,
{
"$subtract": [
{
"$size": "$reports"
},
1
]
}
]
}
]
}
}
},
{
"$project": {
firstElem: false
}
},
{
"$merge": {
"into": "collection",
"on": "_id",
"whenMatched": "replace"
}
}
])
Here is the Mongo playground for your reference.