MongoDB: group, and then counting different values - mongodb

Question

I have a list of books. I would like to get the number of genres that the authors wrote, also I would like to add to the results which are those genres. My database looks like this:
{"_id": ObjectID("1), "title": "Harry Potter", "year": NumberInt(2000), "author": "JK. Rowling",
"genres": "Fantasy"},
"_id": ObjectID("2"), "title": "Harry Potter 99", "year": NumberInt(2020), "author": "JK. Rowling",
"genres": "Drama"}, "_id": ObjectID("2"), "title": "Harry Potter", "year": NumberInt(2000), "author": "JK. Rowling",
"genres": "Drama"}, {...}
So, my code so far looks like this:
phase1 = {$group : {"_id" : "$author"}, "countgenres" : {$sum : 1}}
phase2 = {$addFields : "genres"}}
phase3 = {$sort : {"numgenres" : -1}}
steps = [phase1, phase2, phase3]
db.database.aggregate(steps)
This is not working for me, so I would like if someone could help me to write a correct code to do this. The result should look like this:
{
"_id" : "JK. Rowling",
"countgenres" : 4,
"genres" : [
"War",
"Fantasy",
"Drama",
"Crime"
]
}
Thanks.

Answer 1

you cannot do that directly in the $group stage. Instead of that, you have to use an $addFields stage and use $reduce and $setUnion, to concat arrays without doubles.
Then you can just add a field with your new array size, and do the $sort.
db.collection.aggregate([
{
$group: {
_id: "$author",
genres: {
$push: "$genres"
}
}
},
{
$addFields: {
genres: {
"$reduce": {
"input": "$genres",
"initialValue": [],
"in": {
$setUnion: [
"$$value",
"$$this"
]
}
}
}
}
},
{
$addFields: {
countGenres: {
$size: "$genres"
}
}
}
])
You can test here

Answer 2

Try this query:
db.collection.aggregate([
{
"$match": {
"author": "JK. Rowling"
}
},
{
"$group": {
"_id": "$author",
"genres": {
"$addToSet": "$genres"
}
}
},
{
$addFields: {
genres: {
"$reduce": {
"input": "$genres",
"initialValue": [],
"in": {
$setUnion: [
"$$value",
"$$this"
]
}
}
}
}
},
{
"$project": {
"countgenres": {
"$size": "$genres"
},
"genres": 1
}
}
])
First stage is $match by the author.
Then group and I've used $addToSet to avoid repeated values.
After that, with $addFields values are merged.
After the group, the field genres is output like this:
"genres": [
[
"War",
"Fantasy",
"Drama",
"Crime"
],
[
"War",
"Fantasy",
"Drama"
]
]
And to merge is neccessary to do $setUnion.
And, the last step is count the array size and output the values you want.
Example here