MongoDB aggregation: how to find out if an array contains multiple elements - mongodb

Question

I have following data.
[
{
"_id": 1,
"array": [
{
"name": "name1",
"nestedArray": [
{
"key": "key1",
"value": "value1"
},
{
"key": "key2",
"value": "value2"
}
]
},
{
"name": "name2",
"nestedArray": [
{
"key": "key1",
"value": "abc"
},
{
"key": "key2",
"value": "value2"
}
]
}
]
}
]
playground
I want to keep the entry whose nestedArray contains 2 matching elements and remove others. The 2 elements are below
{
"key": "key1",
"value": "abc"
},
{
"key": "key2",
"value": "value2"
}
So that the result will be below
[
{
"_id": 1,
"array": [
{
"name": "name2",
"nestedArray": [
{
"key": "key1",
"value": "abc"
},
{
"key": "key2",
"value": "value2"
}
]
}
]
}
]
The one whose name="name1" is removed since it has only one matching element.
Feels like we could use $elemMatch but couldn't figure it out.

Answer 1

First, Unwind the array so that you can easily access the nestedArray .
Second, use $all and $elementMatch on nestedArray
db.collection.aggregate([
{
$unwind: "$array"
},
{
$match: {
"array.nestedArray": {
$all: [
{
"$elemMatch": {
key: "key1",
value: "abc"
}
},
{
"$elemMatch": {
key: "key2",
value: "value2"
}
}
]
}
}
},
{
$group: {
_id: "$_id",
array: {
"$push": "$array"
}
}
}
])
playground

Answer 2

You didn't really specify how you input looks but the strategy will stay the same regardless, We will iterate over the array with $filter and match only documents that their subarray's size is equal to a filtered subarray based on the given input, like so:
// the current input structure.
inputArray = [
{
"key": "key1",
"value": "abc"
},
{
"key": "key2",
"value": "value2"
}
];
db.collection.aggregate([
{
$project: {
array: {
$filter: {
input: "$array",
as: "element",
cond: {
$and: [
{
$eq: [
{
$size: {
$filter: {
input: "$$element.nestedArray",
as: "nested",
cond: {
"$setIsSubset": [
[
"$$nested"
],
inputArray
]
}
}
},
},
{
$size: "$$element.nestedArray"
}
]
},
{ // this is required for an exact match otherwise subsets are possible, if you wan't to allow it delete this equality completly.
$eq: [
{
$size: "$$element.nestedArray"
},
{
$size: inputArray
}
]
}
]
}
}
}
}
}
])
Again if the input is coming in a different structure you'll have to change the $eq conditions