Find percent in mongo - mongodb

Question

I have a collection with 2 docs like below.
{
_id:1,
Score: 30,
Class:A,
School:X
}
{
Score:40,
Class:A,
School:Y
}
I need help in writing query to find out percentage of score like below
{
School:X,
Percent:30/70
}
{
School:Y
Percent:40/70
}

Answer 1

This input:
var r =
[
{"school":"X", "class":"A", "score": 30}
,{"school":"Y", "class":"A", "score": 40}
,{"school":"Z", "class":"A", "score": 20}
,{"school":"Y", "class":"B", "score": 50}
,{"school":"Z", "class":"B", "score": 17}
];
run through this pipeline:
db.foo.aggregate([
// Use $group to gather up the class and save the inputs via $push
{$group: {_id: "$class", tot: {$sum: "$score"}, items: {$push: {score:"$score",school:"$school"}}} }
// Now we have total by class, so just "reproject" that array and do some nice
// formatting as requested:
,{$project: {
items: {$map: { // overwrite input array $items; this is OK
input: "$items",
as: "z",
in: {
school: "$$z.school",
pct: {$concat: [ {$toString: "$$z.score"}, "/", {$toString:"$tot"} ]}
}
}}
}}
]);
produces this output, where _id is the Class:
{
"_id" : "A",
"items" : [
{"school" : "X", "pct" : "30/90"},
{"school" : "Y", "pct" : "40/90"},
{"school" : "Z", "pct" : "20/90"}
]
}
{
"_id" : "B",
{"school" : "Y", "pct" : "50/67"},
{"school" : "Z", "pct" : "17/67"}
]
}
From here you can $unwind if you wish.