How to group by multiple object elements - mongodb

Question

I have sample data like below
[
{
brand:"iphone",
category:"mobile"
},
{
brand:"iphone",
category:"laptop"
},
{
brand:"lenova",
category:"laptop"
}
]
and expecting result as
[
{
brand:"iphone",
count:2
},
{
brand:"lenova",
count:1
},
{
category:"laptop",
count:2
},
{
category:"mobile",
count:1
}
]
Here I want group by same object with multiple fields and get there count. Can any one please let me how to do that in the mongoose.

Answer 1

I am not familiarised with Mongoose. Just tried in Mongoshell
db.getCollection('test').aggregate([
{
$group:{
_id:"$brand",
brand:{$first:"$brand"},
category:{$first:"$category"}
}
},
{$project:{_id:0}}
])

Answer 2

Possible only by using two queries.
Group By Brand
db.getCollection('pages').aggregate([
{
$group: {_id: "$brand", category: { $push: "$category" }}
},
{
$project : {
_id : 0, brand : "$_id", count : {$size : "$category"}
}
},
{ $unwind: { path: "$category", preserveNullAndEmptyArrays: true } }
])
Result:-
/* 1 */
{
"brand" : "lenova",
"count" : 1
}
/* 2 */
{
"brand" : "iphone",
"count" : 2
}
Group By Category
db.getCollection('pages').aggregate([
{
$group: {
_id: "$category", brand: { $push: "$brand" },
}
},
{
$project : {
_id : 0, category : "$_id", count : {$size : "$brand"}
}
},
{ $unwind: { path: "$brand", preserveNullAndEmptyArrays: true } },
])
Result:-
/* 1 */
{
"category" : "laptop",
"count" : 2
}
/* 2 */
{
"category" : "mobile",
"count" : 1
}
Merge them for the required output.

Answer 3

We can use $facet to run parallel aggregation on data.
The following query can get us the expected output:
db.collection.aggregate([
{
$facet:{
"brand_group":[
{
$group:{
"_id":"$brand",
"brand":{
$first:"$brand"
},
"count":{
$sum:1
}
}
},
{
$project:{
"_id":0
}
}
],
"category_group":[
{
$group:{
"_id":"$category",
"category":{
$first:"$category"
},
"count":{
$sum:1
}
}
},
{
$project:{
"_id":0
}
}
]
}
},
{
$project:{
"array":{
$concatArrays:["$brand_group","$category_group"]
}
}
},
{
$unwind:"$array"
},
{
$replaceRoot:{
"newRoot":"$array"
}
}
]).pretty()
Data set:
{
"_id" : ObjectId("5da5c0d0795c8651a7f508c2"),
"brand" : "iphone",
"category" : "mobile"
}
{
"_id" : ObjectId("5da5c0d0795c8651a7f508c3"),
"brand" : "iphone",
"category" : "laptop"
}
{
"_id" : ObjectId("5da5c0d0795c8651a7f508c4"),
"brand" : "lenova",
"category" : "laptop"
}
Output:
{ "brand" : "lenova", "count" : 1 }
{ "brand" : "iphone", "count" : 2 }
{ "category" : "laptop", "count" : 2 }
{ "category" : "mobile", "count" : 1 }