Variable set of attributes $facets MongoDB [duplicate] - mongodb

Question

I have the following document structure in MongoDB:
{
// other keys,
tags: [
tagA: "red",
tagB: "green"
]
},
{
// other keys,
tags: [
tagA: "orange",
tagB: "green",
tagC: "car"
]
}
I want to perform a $facets search that gives me the following output (name of each tag + values that occur on that tag + count of these value):
{
[
tagA: {
red: 1,
orange: 1
},
tagB: {
green: 2
},
tagC: {
car: 1
}
]
}
The tricky part is that the facets are unknown upfront (they can vary), and every tutorial I found only works for a predefined set of facets.
Is it possible?
P.S.: how to get the output of this to come alongside with a given query? So that the return is something like:
{
queryResults: [all the results, as in a normal query],
facets: [result showed in accepted answer]
}

Answer 1

If you consider having this as input (i've added bracket around object in your array) :
[
{
tags: [
{
tagA: "red"
},
{
tagB: "green"
}
]
},
{
tags: [
{
tagA: "orange"
},
{
tagB: "green"
},
{
tagC: "car"
}
]
}
]
You could then do an aggregation pipeline as follow :
db.collection.aggregate([
{
"$unwind": "$tags"
},
{
"$addFields": {
"kv": {
"$objectToArray": "$tags"
}
}
},
{
"$unwind": "$kv"
},
{
"$group": {
"_id": {
key: "$kv.k",
value: "$kv.v"
},
"count": {
"$sum": 1
}
}
},
{
"$group": {
"_id": "$_id.key",
"value": {
"$push": {
"k": "$_id.value",
"v": "$count"
}
}
}
},
{
$project: {
val: [
{
k: "$_id",
v: {
"$arrayToObject": "$value"
}
}
]
}
},
{
$project: {
res: {
"$arrayToObject": "$val"
}
}
},
{
$replaceRoot: {
newRoot: "$res"
}
}
])
It would give you this result :
[
{
"tagA": {
"orange": 1,
"red": 1
}
},
{
"tagB": {
"green": 2
}
},
{
"tagC": {
"car": 1
}
}
]
You can see this on mongoplayground : https://mongoplayground.net/p/FZbM-BGJRBm
Hope this answer your question.
Detailled explanation :
I use $unwind on the tags field in order to get one object per object in tags array.
I use $objectToArray to get keys (tagsA, tagsB) as values.
$unwind to go from an array to objets.
$group with $sum accumulator to calculate the occurence of each unique combination.
$group by tagsA,tagsB, etc with $push accumulator to add value in array (will be usufull afterwards)
$arrayToObject to go from array to object
Same
$replaceRoot to display results better.
If you want to understand more each step, consider reading mongo doc of each pipeline aggregator i used. You can also use the mongoplayground link above, delete some code to see what happens after each step.