Get the maximum of difference of two fields in mongodb - mongodb

Question

I'm newbie to mongoDB. Here I face with an error while I want to get the maximum value of the difference of two fields.
Here is the structure of data saved in database:
{
"_id" : ObjectId("52b3833bd3e98582d2bfb628"),
"author" : {
"name" : "Graydon Hoare",
"email" : "graydon#gmail.com"
},
"title" : "Why Rust ditched pure functions",
"body" : "sth",
"url" : "http://thread.gmane.org/gmane.comp.lang.rust.devel/3674/focus=3855",
"date" : ISODate("2013-04-30T13:23:00.000Z"),
"starred" : 105,
"ratings" : [
3,
5,
3,
2,
4,
1,
3,
3,
3,
2,
3
],
"comments" : [
{
"user" : "tr0lltherapy",
"upVotes" : 18,
"downVotes" : 2,
"text" : "sth",
"replies" : [
{
"user" : "thedeemon",
"upVotes" : 10,
"downVotes" : 0,
"text" : "sth"
},
{
"user" : "mcandre",
"upVotes" : 0,
"downVotes" : 5,
"text" : "sth"
},
{
"user" : "lacosaes0",
"upVotes" : 30,
"downVotes" : 6,
"text" : "Particular emphasis on memory safety."
}
]
},
{
"user" : "hypster",
"upVotes" : 30,
"downVotes" : 2,
"text" : "tl;dr everybody was type-fu fighting",
"replies" : [
{
"user" : "homoiconic",
"upVotes" : 15,
"downVotes" : 0,
"text" : "Here comes the Big Boss, Hu! Simon Peyton-Jones."
}
]
}
],
"tags" : [
"Rust",
"Computer",
"Programming"
],
"draft" : true,
"published" : true
}
What I want is to get the value of maximum of the subtract of upVotes and downVotes in replies and comments.
db.getCollection('links').aggregate([
{$project: {
_id: "$author",
maxVote: $max: {
$subtract: ["$comments.upVotes", "$comments.downVotes"]
}
}
}
])
I don't know how to fix it!

Answer 1

You can use $map to get the difference for each comment (using $subtract) and then run $max on the output from mapped comments. Additionally you need another nested $max to get the differences from replies, try:
db.col.aggregate([
{
$project: {
maxVote: {
$max: {
$map: {
input: "$comments",
as: "comment",
in: {
$max: {
$concatArrays: [
[ { $subtract: [ "$$comment.upVotes", "$$comment.downVotes" ] } ],
{
$map: {
input: "$$comment.replies",
as: "reply",
in: { $subtract: [ "$$reply.upVotes", "$$reply.downVotes" ] }
}
}
]
}
}
}
}
}
}
}
])
prints:
{ "_id" : ObjectId("..."), "maxVote" : 28 }