wrong counting with $filter - node.js

Question

My db cofiguration looks like:
{
"_id" : ObjectId("5ece47aa6510a611b47aac5a"),
"boats" : [
{
"_id" : ObjectId("5ece47aa6510a611b47aac6e"),
"model" : "Dufour",
"year" : 2019,
"about" : [
{
"_id" : ObjectId("5ece47aa6510a611b47aac71"),
"Capacity" : 14,
"characteristics" : [
{
"_id" : ObjectId("5ece47aa6510a611b47aac73"),
"fuel" : "petrol",
"fuelCap" : 200
},
{
"_id" : ObjectId("5ece47aa6510a611b47aac73"),
"fuel" : "petrol",
"fuelCap" : 120
},
]
},
{
"_id" : ObjectId("5ece47aa6510a611b47aac71"),
"Capacity" : 8,
"characteristics" : [
{
"_id" : ObjectId("5ece47aa6510a611b47aac73"),
"fuel" : "benzin",
"fuelCap" : 180
},
{
"_id" : ObjectId("5ece47aa6510a611b47aac73"),
"fuel" : "petrol",
"fuelCap" : 100
},
]
},
{...},
{...},
]
}
Now i am trying to count the number of boats which have "fuel" : "petrol", so i use the code bellow:
router.get('/boat', async(req, res)=>{
try{
const fuelData = await Boat.aggregate([
{
$project: {
fuelData: {
$filter: {
input: "$boats",
as: "boats",
cond: {
$filter:{
input:"$$boats.about",
as:"about",
cond:{
$filter:{
input:"$$about.characteristics",
as:"characteristics",
cond:{
$eq:["$$activity1.activity.type", "STILL"]
}
}
}
}
}
}
}
}
},
{
$project: {
boatsCount: {$size : "$fuelData" }
}
}
])
res.status(201).send(fuelData)
}catch(e){
res.send(e)
}
})
The problem is that return wrong number of boatCount. And it seems like it returns the number of the boats which are inside the db. Any help how to count correctly the boats which have "fuel" : "petrol"?
Is there anything wrong in my code?

Answer 1

https://mongoplayground.net/p/D0FEhTMJEJ1
Hope this is what you need.
The sample data you have provided is missing a ] & }.
So I added 1 additional boat with 2 about.
db.collection.aggregate([
{
$match: {
"boats.about.characteristics.fuel": "petrol"
}
},
{
$unwind: "$boats"
},
{
$unwind: "$boats.about"
},
{
$match: {
"boats.about.characteristics.fuel": "petrol"
}
},
{
$group: {
_id: null,
count: {
$sum: 1
}
}
}
])