Split arrays in set mongodb - arrays

Question

I've been trying to make a set of strings < or arrays > but my main issue is that I can't use $unwind because of the other data.
To make it more clear I'll show an example:
This is how part of my documents look like:
{
"key": [
"a",
"b",
"c"
]
},
{
"key": [
"b",
"d"
]
},
{
"key": [
"e"
]
}
And I want to get an array like this:
{
"keys": [
"a",
"b",
"c",
"d",
"e"
]
}
or this:
{
"keys": [
["a"],
["b"],
["c"],
["d"],
["e"]
]
}
I just need unique values in there.
It would be so easy if I could use $unwind and then $addToSet. But as I said before I can't because of the other data that I need.
I was trying to use $cond inside the $addToSet but I don't know how to iterate through an array.
This pipeline doesn't work ->
db.collection.aggregate([
{
"$group": {
"_id": null,
"keys": {
"$addToSet": {
"$concatArrays": [
"$key",
"$this"
]
}
},
}
}
])
Maybe something like this:
{
"$group": {
"_id": null,
"keys": {
"$addToSet": {
"$cond": {
if: {
"$gt": [
{"$size": "$key"},
1
]
},
then: "<SPLIT THE ARRAY IN THE AMOUNT OF ELEMENTS>",
else: "$key"
}
}
}
}
}
I tried this but it doesn't work either.
db.collection.aggregate([
{
"$project": {
"keys": {
"$map": {
"input": {
"$range": [
0,
{
"$size": "$key"
},
1
]
},
as: "index",
in: {
$slice: [
"$key",
"$$index",
1
]
}
}
}
}
},
{
"$group": {
"_id": null,
"keys": {
"$addToSet": {
"$concatArrays": [
"$keys"
]
}
},
}
}
])
Here is my code snippet: https://mongoplayground.net/p/sMENXyrbQgI
Could someone please guide me! I'll appreciate it so much.

Answer 1

So the idea is to use $setUnion along with $reduce to pick unique values from the array:
Solution in MongoPlayground
Try this query:
db.collection.aggregate([
{
$group: {
_id: null,
"keys": { $push: "$key" }
}
},
{
$addFields: {
"keys": {
$reduce: {
input: "$keys",
initialValue: [],
in: {
$setUnion: ["$$value", "$$this"]
}
}
}
}
}
])
Output:
{
"_id" : null,
"keys" : [
"a",
"b",
"c",
"d",
"e"
]
}
Test data in collection:
[
{
"key": [
"a",
"b",
"c"
]
},
{
"key": [
"b",
"d",
"a",
]
},
{
"key": [
"e",
"b"
]
}
]