Create an unique list of multiple arrays inside a subdocument. - mongodb

Question

I would like to create an unique list of array values inside a subdocument.
Document:
{
"_id" : ObjectId("5aee0e3c059638093b69c8b3"),
"firstname" : "John",
"lastname" : "Doe",
"websites" : [
{
"_id" : ObjectId("123"),
"key" : "website2",
"url" : "www.xxx.com",
"tags" : [
"php",
"python",
"java"
]
},
{
"_id" : ObjectId("456"),
"key" : "website2",
"url" : "www.yyy.com",
"tags" : [
"java",
"php"
]
},
{
"_id" : ObjectId("789"),
"key" : "website3",
"url" : "www.zzz.com",
"tags" : [
"java",
"html",
"css"
]
}
]
}
Expected output:
{
"_id" : ObjectId("5aee0e3c059638093b69c8b3"),
"firstname" : "John",
"lastname" : "Doe",
"unique_tags": [
"java",
"php",
"python",
"html",
"css",
],
"websites" : [
{
"_id" : ObjectId("123"),
"key" : "website2",
"url" : "www.xxx.com",
"tags" : [
"php",
"python",
"java"
]
},
{
"_id" : ObjectId("456"),
"key" : "website2",
"url" : "www.yyy.com",
"tags" : [
"java",
"php"
]
},
{
"_id" : ObjectId("789"),
"key" : "website3",
"url" : "www.zzz.com",
"tags" : [
"java",
"html",
"css"
]
}
]
}
It looks like Mongo has a distinct functionality, but this does not work inside an aggregate query (right?!).
Also tried to unwind on websites.tags and use the addToSet functionality but it also hasn't the right output.
Any ideas?

Answer 1

You can try below aggregation:
db.col.aggregate([
{
$addFields: {
unique_tags: {
$reduce: {
input: {
$concatArrays: {
$map: {
input: "$websites",
as: "website",
in: "$$website.tags"
}
}
},
initialValue: [],
in: { $setUnion : ["$$value", "$$this"]}
}
}
}
}
])
To flatten an array of arrays (websites -> tags) you can use $map with $concatArrays. Then you'll get an array of all tags from all websites. To get only unique values you can use $reduce with $setUnion (which drops duplicates).