Filtering results in mongodb - arrays

Question

maybe a simple question for experimented developers with MongoDB, but I'm not getting a solution:
My json with "Stations" collection:
{
"code": "XX",
"variables": [
{
"code": 1,
"items": [
{
"value": 81
},
{
"value": 77
}
]
},
{
"code":2,
"items": [
{
"value": 33
}
]
}
]
}
....
I want to filter "Station" collection to only get variable with code 1 and item with value 81, obtaining something similar to this:
{
"code": "XX",
"variables": [
{
"code": 1,
"items": [
{
"value": 81
}
]
}
]
}
Due json contains arrays in different levels, my approach (mongo shell) was:
db.stations.find(
{"code": "XX"},
{
"variables":
{ $elemMatch:
{
"code": 1,
"items":
{ $elemMatch:
{
"value": 81
}
}
}
}
}
)
But that's getting all items of the same level of 'value: 81', not only this.
Any idea? I also tried to do something with "aggregate" operator and $redact, but no result...thanks!

Answer 1

As per mongo $elemMatch documentation
The $elemMatch operator matches documents that contain an array field with at least one element that matches all the specified query criteria.
hence using $elemMatch it match in items.value:81 and return whole matching items array like below query
db.stations.find({
"code": "XX"
}, {
"variables": {
"$elemMatch": {
"code": 1
}
},
"variables": {
"$elemMatch": {
"items": {
"$elemMatch": {
"value": 81
}
}
}
}
}).pretty()
This return items.value:81 and items.value:77 because of elemMatch match one elements in array. Or same if used in project as below it shows same result like above query
db.stations.find({
"code": "XX",
"variables": {
"$elemMatch": {
"code": 1
}
},
"variables": {
"$elemMatch": {
"items": {
"$elemMatch": {
"value": 81
}
}
}
}
}, {
"code": 1,
"variables.code.$": 1
}).pretty()
So If you find your expected output then you should use mongo aggregation as below :
db.stations.aggregate({
"$match": {
"code": "XX",
"variables.code": 1
}
}, {
"$unwind": "$variables"
}, {
"$unwind": "$variables.items"
}, {
"$match": {
"variables.items.value": 81
}
}, {
"$group": {
"_id": "$code",
"data": {
"$push": "$variables"
}
}
}, {
"$project": {
"code": "$_id",
"variables": "$data",
"_id": 0
}
}).pretty()