MongoDB Trasnform string array to string concatenated in alphabetical order - mongodb

Question

Playground
Lets say I have this collection:
[
{ "Topics": [ "a", "b" ] },
{ "Topics": [ "x", "a" ] },
{ "Topics": [ "k", "c", "z" ] }
]
I want to transform this string array to a single string with the itens of it in alphabetical order. The result would be:
[
{ Topic: "a/b"},
{ Topic: "a/x"},
{ Topic: "c/k/z"}
]
How can I project this result? Using Map? Reduce?
I have Mongo 5.0
Playground
cheers

Answer 1

just found the solution after some tries...
Just A Unwind, Sort, Group, Project with Reduce made the job...
Data
[
{
"Topics": [
"a",
"b"
]
},
{
"Topics": [
"x",
"a"
]
},
{
"Topics": [
"k",
"c",
"z"
]
}
]
Query
db.collection.aggregate([
{
"$unwind": "$Topics"
},
{
"$sort": {
"Topics": 1
}
},
{
"$group": {
"_id": "$_id",
Topics: {
"$push": "$Topics"
}
}
},
{
"$project": {
Topic: {
$reduce: {
input: "$Topics",
initialValue: "1T1",
in: {
$concat: [
"$$value",
"/",
"$$this"
]
}
}
}
}
}
])
Result:
[
{
"Topic": "1T1/a/x",
"_id": ObjectId("5a934e000102030405000001")
},
{
"Topic": "1T1/c/k/z",
"_id": ObjectId("5a934e000102030405000002")
},
{
"Topic": "1T1/a/b",
"_id": ObjectId("5a934e000102030405000000")
}
]

Answer 2

The common way to do this is
unwind
sort
group by id
reduce to 1 string
Bellow is a way to not unwind all collection but do a "local unwind".
Query
lookup with a dummy collection of 1 empty document [{}]
(this is "trick" that allows us to use stage operators like sort inside 1 document array) you need that collection in your database
unwind topics, sort them, group in 1 array, reduce them and create 1 string
we will have only 1 joined document (the transformed root document),
we replace the root with that
remove the "/" from start (it could be done on the reduce stage also)
added one extra case where topics are empty array to return ""
Test code here
db.topics.aggregate([
{
"$lookup": {
"from": "dummy",
"let": {
"topics": "$Topics"
},
"pipeline": [
{
"$set": {
"Topics": "$$topics"
}
},
{
"$unwind": {
"path": "$Topics"
}
},
{
"$sort": {
"Topics": 1
}
},
{
"$group": {
"_id": null,
"Topics": {
"$push": "$Topics"
}
}
},
{
"$project": {
"_id": 0
}
},
{
"$set": {
"Topics": {
"$reduce": {
"input": "$Topics",
"initialValue": "",
"in": {
"$let": {
"vars": {
"s": "$$value",
"t": "$$this"
},
"in": {
"$concat": [
"$$s",
"/",
"$$t"
]
}
}
}
}
}
}
}
],
"as": "joined"
}
},
{
"$replaceRoot": {
"newRoot": {
"$cond": [
{
"$eq": [
"$joined",
[]
]
},
{
"Topics": ""
},
{
"$arrayElemAt": [
"$joined",
0
]
}
]
}
}
},
{
"$set": {
"Topics": {
"$cond": [
{
"$gt": [
{
"$strLenCP": "$Topics"
},
0
]
},
{
"$substrCP": [
"$Topics",
1,
{
"$strLenCP": "$Topics"
}
]
},
""
]
}
}
}
])