mongoDB count deeply nested array elements filtered by and condition - mongodb

Question

I am attempting to count elements in 3x nested array , I have some progress , but struggling to filter based on lang:"EN" condition inside the 2x $reduced 1x filter :
Here is example document:
{
"_id": ObjectId("5c05984246a0201286d4b57a"),
f: "x",
"_a": [
{
"_onlineStore": {}
},
{
"_p": [
{
"pid": 1,
"s": {
"a": {
"t": [
{
id: 1,
"dateP": "20200-09-20",
lang: "EN"
},
{
id: 2,
"dateP": "20200-09-20",
lang: "En"
}
]
},
"c": {
"t": [
{
id: 3,
lang: "en"
},
{
id: 4,
lang: "En"
},
{
id: 5,
"dateP": "20300-09-23"
}
]
}
},
h: "Some data"
}
]
}]
}
And here is my attempt ( just need to filter only the elements with lang:"EN"
db.collection.aggregate([
{
$project: {
res: {
$reduce: {
input: "$_a",
initialValue: [],
in: {
$concatArrays: [
"$$value",
{
"$cond": {
"if": {
"$eq": [
{
"$type": "$$this._p"
},
"array"
]
},
"then": {
$reduce: {
input: "$$this._p",
initialValue: [],
in: {
$concatArrays: [
"$$value",
{
"$filter": {
"input": {
"$objectToArray": "$$this.s"
},
"as": "f",
"cond": {
"$eq": [
"$$f.k",
"c"
]
}
}
}
]
}
}
},
"else": []
}
}
]
}
}
}
}
},
{
$unwind: "$res"
},
{
$unwind: "$res.v.t"
},
{
$count: "Total"
}
])
I need to count all _a[]._p[].s.c.t[] where lang:"EN","en","En" , note at object s there is multiple nested elements c , a , d , etc , only the c need to be counted where lang:"EN" , I managed to filter only the "c" but struggling to add the lang:"EN","en","En" inside the $filter.cond , can anybody help here?
Expected playground output is:
{count:7}
I can add final $match condition and clear the lang:EN , but I am wondering if there is better option to be done inside the reduce/reduce/objectToArray/cond and avoid the $unwind's?
Playgorund

Answer 1

One option to avoid $unwind is:
db.collection.aggregate([
{$match: {"_a._p.s.c.t": {$elemMatch: {lang: {$in: ["EN", "En", "en"]}}}}},
{$project: {
res: {$reduce: {
input: "$_a._p.s.c.t",
initialValue: [],
in: {$concatArrays: ["$$value", "$$this"]}
}}
}},
{$project: {
res: {$reduce: {
input: "$res",
initialValue: 0,
in: {$sum: [
"$$value",
{$size: {$filter: {
input: "$$this",
as: "inner",
cond: {$in: ["$$inner.lang", ["EN", "En", "en"]]}
}}}
]}
}}
}},
{$group: {_id: 0, count: {$sum: "$res"}}}
])
See how it works on the playground example