Grouping into array in MongoDB - mongodb

Question

I have MongoDB collection with below documents:
[
{
"productType":"Bike",
"company":"yamaha",
"model":"y1"
},
{
"productType":"Bike",
"company":"bajaj",
"model":"b1"
},
{
"productType":"Bike",
"company":"yamaha",
"model":"y1"
},
{
"productType":"Car",
"company":"Maruti",
"model":"m1"
},
{
"productType":"Bike",
"company":"yamaha",
"model":"y2"
},
{
"productType":"Car",
"company":"Suzuki",
"model":"s1"
}
]
I want my output to be like :
{
"productType": [
{
"name": "Bike",
"count": 4,
"companies": [
{
"name": "Yamaha",
"count": 3,
"models": [
{
"name": "y1",
"count": 2
},
{
"name": "y2",
"count": 1
}
]
},
{
"name": "Bajaj",
"count": 1,
"models": [
{
"name": "b1",
"count": 1
}
]
}
]
},
{
"name": "Car",
"count": 2,
"companies": [
{
"name": "Maruti",
"count": 1,
"models": [
{
"name": "m1",
"count": 1
}
]
},
{
"name": "Suzuki",
"count": 1,
"models": [
{
"name": "s1",
"count": 1
}
]
}
]
}
]
}
I am not able to understand how to create arrays inside existing array using $push. I know we can create an array using $push but how to create array of array with it ?
In future, I might want to add "metaData" field also along with name and count.

Answer 1

You have to run multiple $group stages, one for each level:
db.collection.aggregate([
{
$group: {
_id: { company: "$company", productType: "$productType", model: "$model" },
count: { $sum: 1 }
}
},
{
$group: {
_id: { productType: "$_id.productType", company: "$_id.company" },
models: { $push: { name: "$_id.model", count: "$count" } },
count: { $sum: "$count" }
}
},
{
$group: {
_id: "$_id.productType",
companies: { $push: { company: "$_id.company", models: "$models", count: "$count" } },
count: { $sum: "$count" }
}
},
{ $set: { name: "$_id", _id: "$$REMOVE" } },
{
$group: {
_id: null,
productType: { $push: "$$ROOT" }
}
}
])
Mongo Playground

Answer 2

Try this:
db.testCollection.aggregate([
{
$group: {
_id: {
name: "$productType",
company: "$company",
model: "$model"
},
count: { $sum: 1 }
}
},
{
$group: {
_id: {
name: "$_id.name",
company: "$_id.company"
},
count: { $sum: "$count" },
models: {
$push: {
name: "$_id.model",
count: "$count"
}
}
}
},
{
$group: {
_id: { name: "$_id.name" },
count: { $sum: "$count" },
companies: {
$push: {
name: "$_id.company",
count: "$count",
models: "$models"
}
}
}
},
{
$group: {
_id: null,
productType: {
$push: {
name: "$_id.name",
count: "$count",
companies: "$companies"
}
}
}
},
{
$project: { _id: 0 }
}
]);
Output:
{
"productType" : [
{
"name" : "Car",
"count" : 2,
"companies" : [
{
"name" : "Suzuki",
"count" : 1,
"models" : [
{
"name" : "s1",
"count" : 1
}
]
},
{
"name" : "Maruti",
"count" : 1,
"models" : [
{
"name" : "m1",
"count" : 1
}
]
}
]
},
{
"name" : "Bike",
"count" : 4,
"companies" : [
{
"name" : "yamaha",
"count" : 3,
"models" : [
{
"name" : "y2",
"count" : 1
},
{
"name" : "y1",
"count" : 2
}
]
},
{
"name" : "bajaj",
"count" : 1,
"models" : [
{
"name" : "b1",
"count" : 1
}
]
}
]
}
]
}