MongoDB $filter nested array by date does not work - mongodb

Question

I have a document with a nested array which looks like this:
[
{
"id": 1,
data: [
[
ISODate("2000-01-01T00:00:00Z"),
2,
3
],
[
ISODate("2000-01-03T00:00:00Z"),
2,
3
],
[
ISODate("2000-01-05T00:00:00Z"),
2,
3
]
]
},
{
"id": 2,
data: []
}
]
As you can see, we have an array of arrays. For each element in the data array, the first element is a date.
I wanted to create an aggregation pipeline which filters only the elements of data where the date is larger than a given date.
db.collection.aggregate([
{
"$match": {
"id": 1
}
},
{
"$project": {
"data": {
"$filter": {
"input": "$data",
"as": "entry",
"cond": {
"$gt": [
"$$entry.0",
ISODate("2000-01-04T00:00:00Z")
]
}
}
}
}
}
])
The problem is that with $gt, this just returns an empty array for data. With $lt this returns all elements. So the filtering clearly does not work.
Expected result:
[
{
"id": 1,
"data": [
[
ISODate("2000-01-05T00:00:00Z"),
2,
3
]
]
}
]
Any ideas?
Playground

Answer 1

I believe the issue is that when you write $$entry.0, MongoDB is trying to evaluate entry.0 as a variable name, when in reality the variable is named entry. You could make use of the $first array operator in order to get the first element like so:
db.collection.aggregate([
{
"$match": {
"id": 1
}
},
{
"$project": {
"data": {
"$filter": {
"input": "$data",
"as": "entry",
"cond": {
"$gt": [
{
$first: "$$entry"
},
ISODate("2000-01-04T00:00:00Z")
]
}
}
}
}
}
])
Mongo playground example

Answer 2

Don't think $$entry.0 work to get the first element of the array. Instead, use $arrayElemAt operator.
db.collection.aggregate([
{
"$match": {
"id": 1
}
},
{
"$project": {
"data": {
"$filter": {
"input": "$data",
"as": "entry",
"cond": {
"$gt": [
{
"$arrayElemAt": [
"$$entry",
0
]
},
ISODate("2000-01-04T00:00:00Z")
]
}
}
}
}
}
])
Sample Mongo Playground

Answer 3

to specify which element in the array you are comparing it is better to use $arrayElemAt instead of $$ARRAY.0. you must pass 2 parameters while using $arrayElemAt, the first one is the array which in your case is $$entry, and the second one is the index which in your case is 0
this is the solution I came up with:
db.collection.aggregate([
{
"$match": {
"id": 1
}
},
{
"$project": {
"data": {
"$filter": {
"input": "$data",
"as": "entry",
"cond": {
"$gt": [
{
"$arrayElemAt": [
"$$entry",
0
]
},
ISODate("2000-01-04T00:00:00Z")
]
}
}
}
}
}
])
playground