compare two collection in mongodb - mongodb

Question

I have two different collection book and music in JSON .First I give a book collection example:
{
"_id" : ObjectId("b1"),
"author" : [
"Mary",
],
"title" : "Book1",
}
{
"_id" : ObjectId("b2"),
"author" : [
"Joe",
"Tony",
"Mary"
],
"title" : "Book2",
}
{
"_id" : ObjectId("b3"),
"author" : [
"Joe",
"Mary"
],
"title" : "Book3",
}
.......
Mary writes 3 books, Joe write 2 books, Tony writes 1 book. Second I give a music collection example:
{
"_id" : ObjectId("m1"),
"author" : [
"Tony"
],
"title" : "Music1",
}
{
"_id" : ObjectId("m2"),
"author" : [
"Joe",
"Tony"
],
"title" : "Music2",
}
.......
Tony has 2 musics, Joe has 1 music, Mary has 0 music.
I hope to get the number of authors who write more books than music.
Thus, Mary(3 > 0) and Joe(2 > 1) should take into consideration, but not Tony(1 < 2). Thus the final result should be 2(Mary and Joe).
I write down following code, but don't know how to compare:
db.book.aggregate([
{ $project:{ _id:0, author:1}},
{ $unwind:"$author" },
{$group:{_id:"$author", count:{$sum:1}}}
]
)
db.music.aggregate([
{ $project:{ _id:0, author:1}},
{ $unwind:"$author" },
{$group:{_id:"$author", count:{$sum:1}}}
]
)
Is it so far right? How to do the following comparison? Thanks.

Answer 1

to solve that problem, we need to use $out phase and store result of both queries in intermediate collection and then use aggregated query to join them ($lookup).
db.books.aggregate([{
$project : {
_id : 0,
author : 1
}
}, {
$unwind : "$author"
}, {
$group : {
_id : "$author",
count : {
$sum : 1
}
}
}, {
$project : {
_id : 0,
author : "$_id",
count : 1
}
}, {
$out : "bookAuthors"
}
])
db.music.aggregate([{
$project : {
_id : 0,
author : 1
}
}, {
$unwind : "$author"
}, {
$group : {
_id : "$author",
count : {
$sum : 1
}
}
}, {
$project : {
_id : 0,
author : "$_id",
count : 1
}
}, {
$out : "musicAuthors"
}
])
db.bookAuthors.aggregate([{
$lookup : {
from : "musicAuthors",
localField : "author",
foreignField : "author",
as : "music"
}
}, {
$unwind : "$music"
}, {
$project : {
_id : "$author",
result : {
$gt : ["$count", "$music.count"]
},
count : 1,
}
}, {
$match : {
result : true
}
}
])
EDIT CHANGES:
used author field instead of _id
added logical statement embeded in document in $project phase
result : { $gt : ["$count", "$music.count"]
Any questions welcome!
Have a fun!