Mongodb Group by get $max and count of max in value and percent of that group - mongodb

Question

I need to a group by on x field and get the max value of other fields. Yes, using $max we can get max repetitive value. But, I also need to get count $max value in percent/count too. In other words, how many times this $max value exist in that group. Kindly help.
example:
db.getCollection("test").aggregate(
[
{ "$match" : { "doc_id" : 1.0 } },
{ "$group" : {
"_id" : { "name" : "$name" },
"total" : { "$sum" : "$amount" },
"l1_max" : { "$max" : "$l1" }
}
},
]
);
Here, I am getting l1_max = 'Computer' . But, I need it as 'Computer - (30%) Total 4/12'
Updated: 20/10/2019
#mickl : Thanks for the answer.
The field l1 is actually a referenced field. In normal find/project or mongoose populate(), it helps to get fields from other collection. Example:
if l1 is of type ObjectId then,
l1: {
_id, "4343434343sdsdsY",
name: "IT"
}
So l1.name will fetch name field from another collection in project/populate function.
I executed following code:
db.getCollection("test").aggregate(
[
{ "$match" : { "doc_id" : 1.0 } },
{ "$group" : {
"_id" : { "name" : "$name" },
"total" : { "$sum" : "$amount" },
"count": { '$sum': 1 },
"l1_max" : { "$max" : "$l1" },
"l1_values": { $push: "$l1" }
}
},
{
$project: {
_id: 1,
total: 1,
l1 : {"_id": "$l1_max", "count": "$count", "percent": { $divide: [ { $size: { $filter: { input: "$l1_values", cond: { $eq: [ "$$this", "$l1_max" ] } } } },"$count"]}}
}
}
]
);
Answer is like below: But I also need referenced name field too.
{
"_id" : {
"name" : "xzy"
},
"total" : 35.0,
"l1" : {
"_id" : "4343920239201W",
"name" : "IT", // **MISSING**
"count" : 4.0,
"percent" : 0.25
}
}
Hope I was clear this time.

Answer 1

You need to capture all l1 values in your group and the calculate the percent using $divide, $filter and $size:
db.getCollection("test").aggregate(
[
{ "$match" : { "doc_id" : 1.0 } },
{ "$group" : {
"_id" : { "name" : "$name" },
"total" : { "$sum" : "$amount" },
"l1_max" : { "$max" : "$l1" },
"l1_values": { $push: "$l1" }
}
},
{
$project: {
_id: 1,
total: 1,
l1_max: 1,
l1_perc: {
$divide: [
{ $size: { $filter: { input: "$l1_values", cond: { $eq: [ "$$this", "$l1_max" ] } } } },
{ $size: "$l1_values" }
]
}
}
}
]
);
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