MongoDb aggregation framework to group elements of inner array - mongodb

Question

I'm using MongoDB aggregation framework trying to transform each document:
{
"all": [
{
"type": "A",
"id": "1"
},
{
"type": "A",
"id": "1"
},
{
"type": "B",
"id": "2"
},
{
"type": "A",
"id": "3"
}
]
}
into this:
{
"unique_type_A": [ "3", "1" ]
}
(final result is a collection of n documents with unique_type_A field)
The calculation consists of returning in an array all the uniques types of entities of type A.
I got stuck with $group step, anyone knows how to do it?

Answer 1

To apply this logic to each document, you can use the following;
db.collection.aggregate([
{
$unwind: "$all"
},
{
$match: {
"all.type": "A"
}
},
{
$group: {
_id: {
"type": "$all.type",
"oldId": "$_id"
},
unique_type_A: {
$addToSet: "$all.id"
}
}
},
{
$project: {
_id: 0
}
}
])
Where we first $unwind, to be able to filter and play with each member of all array. Then we just filter the non type:"A" members. The $group stage has the difference with a complex _id, where we utilize the _id of $unwind result, which refers back to the original document, so that we can group the results per original document. Collecting the id from all array with $addToSet to keep only unique values, and voilà!
And here is the result per document;
[
{
"unique_type_A": [
"3",
"1"
]
},
{
"unique_type_A": [
"4",
"11",
"5"
]
}
]
Check the code interactively on Mongoplayground