MongoDB - Find array index of document in array field - mongodb

Question

I have aggregation that contains array field. This array field contains documents (objects). For these I have some matching criteria and I would like create a new fields named lowestCheapIndex and highestExpensiveIndex, each with array index of matching element.
Matching criteria:
lowestCheapIndex - Should contain lowest array index number of any record item, that has price below 20.
highestExpensiveIndex - Should contain highest array index number of any record item, that has price over 30.
My current aggregation output:
{
'_id': 'Egg shop',
'records': [
{'_id': 1, 'price': 22},
{'_id': 2, 'price': 18},
{'_id': 3, 'price': 34},
{'_id': 4, 'price': 31},
{'_id': 5, 'price': 13},
]
}
Desired output:
{
'_id': 'Egg shop',
'records': [
{'_id': 1, 'price': 22},
{'_id': 2, 'price': 18},
{'_id': 3, 'price': 34},
{'_id': 4, 'price': 31},
{'_id': 5, 'price': 13},
],
'lowestCheapIndex': 1,
'highestExpensiveIndex': 3,
}
Question:
How can i retrieve array index based on my criteria? I found $indexOfArray in docs, but still I am having hard time how it would be used in my case.

Answer 1

You can do following in an aggregation pipeline:
use $map to augment your records array with booleans indicating below 20 and over 30
use $indexOfArray to search for the booleans; For highestExpensiveIndex, reverse the array first to get the index then subtract it from size of array - 1 to get the expected index.
db.collection.aggregate([
{
"$addFields": {
"records": {
"$map": {
"input": "$records",
"as": "r",
"in": {
"_id": "$$r._id",
"price": "$$r.price",
"below20": {
$lt: [
"$$r.price",
20
]
},
"over30": {
$gt: [
"$$r.price",
30
]
}
}
}
}
}
},
{
"$addFields": {
"lowestCheapIndex": {
"$indexOfArray": [
"$records.below20",
true
]
},
"highestExpensiveIndex": {
"$subtract": [
{
"$subtract": [
{
$size: "$records"
},
{
"$indexOfArray": [
{
"$reverseArray": "$records.over30"
},
true
]
}
]
},
1
]
}
}
}
])
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