MongoDB multi level group - mongodb

Question

I have some input data :
Brand | Model | Number
Peugeot | 208 | 1
Peugeot | 4008 | 2
Renault | Clio | 3
Renault | Megane | 4
I would like to get both :
the sum for each brand
the global sum
Here is my expected output :
Brand | Number
Peugeot | 3
Renault | 7
Total | 10
I think I have to create two $group operations and set Total with $literal.
What is the right way to do so ?

Answer 1

As you said this can be done by 2 group bys, so let's start by putting some data in to mongo similar to your example input:
> db.cars.insertMany([
{ "Brand" : "Peugeot", "Model" : "208", "Number": 1 },
{ "Brand" : "Peugeot", "Model" : "4008", "Number": 2 },
{ "Brand" : "Renault", "Model" : "Clio", "Number": 3 },
{ "Brand" : "Renault", "Model" : "Megane", "Number": 4 }
]);
Now we've got all our cars inserted we can then aggregate these using the 2 group aggregation operators:
db.cars.aggregate([
{ $group : { "_id" : "$Brand", "Number" : { $sum : "$Number" }}},
{ $group : { "_id" : null, "Rows" : { $push : { "Brand" : "$$ROOT._id", "Number" : "$Number" } }, "Total" : {$sum : "$Number" } }}
])
This will give us the following output
{
"_id" : null,
"Rows" : [
{
"Brand" : "Renault",
"Number" : 7
},
{
"Brand" : "Peugeot",
"Number" : 3
}
],
"Total" : 10
}
We can then clean it up with a projection
db.cars.aggregate([
{ "$group" : { "_id" : "$Brand", "Number" : { $sum : "$Number" }}},
{ "$group" : { "_id" : null, "Rows" : { $push : { "Brand" : "$$ROOT._id", "Number" : "$Number" } }, "Total" : {$sum : "$Number" } } },
{ "$project" : { "_id" : 0, "Data" : { "$concatArrays" : [ "$Rows", [ { "Brand": { $literal : "Total" }, "Number" : "$Total" } ] ] } } }
])
Giving us the following result
{
"Data" : [
{
"Brand" : "Renault",
"Number" : 7
},
{
"Brand" : "Peugeot",
"Number" : 3
},
{
"Brand" : "Total",
"Number" : 10
}
]
}