Comparing 2 fields from $project in a mongoDB pipeline - mongodb

Question

In a previous post I created a mongodb query projecting the number of elements matching a condition in an array. Now I need to filter this number of elements depending on another field.
This is my db :
db={
"fridges": [
{
_id: 1,
items: [
{
itemId: 1,
name: "beer"
},
{
itemId: 2,
name: "chicken"
}
],
brand: "Bosch",
size: 195,
cooler: true,
color: "grey",
nbMax: 2
},
{
_id: 2,
items: [
{
itemId: 1,
name: "beer"
},
{
itemId: 2,
name: "chicken"
},
{
itemId: 3,
name: "lettuce"
}
],
brand: "Electrolux",
size: 200,
cooler: true,
color: "white",
nbMax: 2
},
]
}
This is my query :
db.fridges.aggregate([
{
$match: {
$and: [
{
"brand": {
$in: [
"Bosch",
"Electrolux"
]
}
},
{
"color": {
$in: [
"grey",
"white"
]
}
}
]
}
},
{
$project: {
"itemsNumber": {
$size: {
"$filter": {
"input": "$items",
"as": "item",
"cond": {
$in: [
"$$item.name",
[
"beer",
"lettuce"
]
]
}
}
}
},
brand: 1,
cooler: 1,
color: 1,
nbMax: 1
}
}
])
The runnable example.
Which gives me this :
[
{
"_id": 1,
"brand": "Bosch",
"color": "grey",
"cooler": true,
"itemsNumber": 1,
"nbMax": 2
},
{
"_id": 2,
"brand": "Electrolux",
"color": "white",
"cooler": true,
"itemsNumber": 2,
"nbMax": 2
}
]
What I expect is to keep only the results having a itemsNumber different from nbMax. In this instance, the second fridge with _id:2 would not match the condition and should not be in returned. How can I modify my query to get this :
[
{
"_id": 1,
"brand": "Bosch",
"color": "grey",
"cooler": true,
"itemsNumber": 1,
"nbMax": 2
}
]

Answer 1

You can put a $match stage with expression condition at the end of your query,
$ne to check both fields should not same
{
$match: {
$expr: { $ne: ["$nbMax", "$itemsNumber"] }
}
}
Playground