Mongodb $expr on array - mongodb

Question

Mongodb: 4.0.13
I'm having troubles in understand and get working $expr with arrays.
Let' start and create a new collection (dbRepeatElement) with following document:
db.testRepeatElement.insert([
{
"data" : {
"FlsResSemires_2" : {
"Sospensione" : [
{
"DataInizio" : 1548806400000,
"DataFine" : 1549065600000,
"Motivazione" : "1"
}
]
}
},
"derived" : {
"DATAFINEANNORIFERIMENTO" : 1609372800000,
"regione190" : "190",
"REGAOEROG" : "190209820300",
"REGASLEROG" : "190209"
}
}
])
In a bigger aggregation, following part is not working:
db.testRepeatElement.aggregate([
{
$match: {
$expr: {
$gt: ["$data.FlsResSemires_2.Sospensione.DataInizio", "$derived.DATAFINEANNORIFERIMENTO"]
}
}
}
])
Result: return a match ( wrong! just check dates)
Reading mongodb documentation seems to be, using combination with arrays, aggregation and $expr does not return expected result and you have to specify with element of the array you want to check, like:
db.testRepeatElement.aggregate([
{
$match: {
$expr: {
$gt: ["$data.FlsResSemires_2.0.Sospensione.DataInizio", "$derived.DATAFINEANNORIFERIMENTO"]
}
}
}
])
Result: return no match (right!)
Question: my requirement is to check every element in the array, so how to solve this, without using $unwind? Why there is this kind of result ?

Answer 1

The $filter aggregation operator is used to do the match operation on array elements. The following aggregation query will result only the Sospensione array elements which match the $gt condition:
db.testRepeatElement.aggregate( [
{
$addFields: {
"data.FlsResSemires_2.Sospensione": {
$filter: {
input: "$data.FlsResSemires_2.Sospensione",
cond: {
$gt: [ "$$this.DataInizio", "$derived.DATAFINEANNORIFERIMENTO" ]
}
}
}
}
},
{
$match: {
$expr: {
$gt: [ { $size: "$data.FlsResSemires_2.Sospensione" }, 0 ]
}
}
}
] ).pretty()