Get percentages with MongoDB aggregate $group - mongodb

Question

I'd like to get percentages from a group pipeline in a MongoDB aggregate.
My data:
{
_id : 1,
name : 'hello',
type : 'big'
},
{
_id : 2,
name : 'bonjour',
type : 'big'
},
{
_id : 3,
name : 'hi',
type : 'short'
},
{
_id : 4,
name : 'salut',
type : 'short'
},
{
_id : 5,
name : 'ola',
type : 'short'
}
My request group by type, and count:
[{
$group : {
_id : {
type : '$type'
},
"count" : {
"$sum" : 1
}
}
}]
Result:
[
{
_id {
type : 'big',
},
count : 2
},
{
_id {
type : 'short',
},
count : 3
}
]
But I'd like to have count AND percentage, like that:
[
{
_id {
type : 'big',
},
count: 2,
percentage: 40%
},
{
_id {
type : 'short',
},
count: 3,
percentage: 60%
}
]
But I've no idea how to do that. I've tried $divide and other things, but without success. Could you please help me?

Answer 1

Well I think percentage should be string if the value contains %
First get you will need to count the number of document.
var nums = db.collection.count();
db.collection.aggregate(
[
{ "$group": { "_id": {"type": "$type"}, "count": { "$sum": 1 }}},
{ "$project": {
"count": 1,
"percentage": {
"$concat": [ { "$substr": [ { "$multiply": [ { "$divide": [ "$count", {"$literal": nums }] }, 100 ] }, 0,2 ] }, "", "%" ]}
}
}
]
)
Result
{ "_id" : { "type" : "short" }, "count" : 3, "percentage" : "60%" }
{ "_id" : { "type" : "big" }, "count" : 2, "percentage" : "40%" }

Answer 2

First find total number of documents in collections using count method and used that count variable to calculate percentage in aggregation like this :
var totalDocument = db.collectionName.count() //count total doc.
used totalDocument in aggregation as below :
db.collectionName.aggregate({"$group":{"_id":{"type":"$type"},"count":{"$sum":1}}},
{"$project":{"count":1,"percentage":{"$multiply":[{"$divide":[100,totalDocument]},"$count"]}}})
EDIT
If you need to this in single aggregation query then unwind used in aggregation but using unwind it creates Cartesian problem check below aggregation query :
db.collectionName.aggregate({"$group":{"_id":null,"count":{"$sum":1},"data":{"$push":"$$ROOT"}}},
{"$unwind":"$data"},
{"$group":{"_id":{"type":"$data.type"},"count":{"$sum":1},
"total":{"$first":"$count"}}},
{"$project":{"count":1,"percentage":{"$multiply":[{"$divide":[100,"$total"]},"$count"]}}}
).pretty()
I recconmed first find out toatal count and used that count in aggregation as per first query.