mongodb: How to build facets from an array of objects - mongodb

Question

What would be the best way to create facets from an array of objects with different attributes for each product.
Example Data:
[
{
"key": 1,
"title": "Product A",
"attrs": [
{
"keyasd": "size",
"value": "small"
},
{
"kedasy": "color",
"value": "red"
}
]
},
{
"key": 2,
"title": "Product B",
"attrs": [
{
"key": "size",
"value": "large"
},
{
"key": "color",
"value": "blue"
}
]
},
{
"key": 3,
"title": "Product C",
"attrs": [
{
"key": "resolution",
"value": "8K"
},
{
"key": "refresh rate",
"value": "60 Hz"
},
]
}
]
The result I would like to get would be something like this:
[
{
"_id": {
"key": "size",
"values" : [
{"title": "small", "count": 1},
{"title": "large", "count": 1}
]
}
},
{
"_id": {
"key": "color",
"values" : [
{"title": "red", "count": 1},
{"title": "blue", "count": 1}
]
}
},
{
"_id": {
"key": "resolution",
"values" : [
{"title": "8K", "count": 1}
]
}
},
{
"_id": {
"key": "refresh rate",
"values" : [
{"title": "60 Hz", "count": 1}
]
}
}
]
I don't know if the result I put is possible, but I need to somehow build it, even if it's individually each facet for each type of attribute that a product can have

Answer 1

db.collection.aggregate([
{
"$unwind": "$attrs"
},
{
"$group": {
"_id": {
k: "$attrs.key",
v: "$attrs.value"
},
"count": { "$sum": 1 }
}
},
{
"$group": {
"_id": "$_id.k",
"values": {
"$push": {
"title": "$_id.v",
"count": "$count"
}
}
}
},
{
"$project": {
"_id": {
key: "$_id",
values: "$values"
}
}
}
])
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