Mongodb use multiple group operator in single aggregation - mongodb

Question

I am using mongodb aggregation for getting counts of different fields. Here are some documents from the mobile collection:-
{
"title": "Moto G",
"manufacturer": "Motorola",
"releasing": ISODate("2011-03-00T10:26:48.424Z"),
"rating": "high"
}
{
"title": "Asus Zenfone 2",
"manufacturer": "Asus",
"releasing": ISODate("2014-10-00T10:26:48.424Z"),
"rating": "high"
}
{
"title": "Moto Z",
"manufacturer": "Motorola",
"releasing": ISODate("2016-10-12T10:26:48.424Z"),
"rating": "none"
}
{
"title": "Asus Zenfone 3",
"manufacturer": "Asus",
"releasing": ISODate("2016-08-00T10:26:48.424Z"),
"rating": "medium"
}
I can find manufacturer and rating counts but this fails:
db.mobile.aggregate([
{
$group: { _id: "$manufacturer", count: { $sum: 1 } }
}, {
$group: { _id: "$rating", count: { $sum: 1 } }
}
])
Output:-
{
"_id" : null,
"count" : 2.0
}
Expected Output something like:-
{
"_id":"Motorola",
"count" : 2.0
}
{
"_id":"Asus",
"count" : 2.0
}
{
"_id":"high",
"count" : 2.0
}
{
"_id":"none",
"count" : 1.0
}
{
"_id":"medium",
"count" : 1.0
}

Answer 1

I believe you are after an aggregation operation that groups the documents by the manufacturer and rating keys, then do a further group on the manufacturer while aggregating the ratings per manufacturer, something like the following pipeline:
db.mobile.aggregate([
{
"$group": {
"_id": {
"manufacturer": "$manufacturer",
"rating": "$rating"
},
"count": { "$sum": 1 }
}
},
{
"$group": {
"_id": "$_id.manufacturer",
"total": { "$sum": 1 },
"counts": {
"$push": {
"rating": "$_id.rating",
"count": "$count"
}
}
}
}
])
Sample Output
/* 1 */
{
"_id" : "Motorola",
"total" : 2,
"counts" : [
{
"rating" : "high",
"count" : 1
},
{
"rating" : "none",
"count" : 1
}
]
}
/* 2 */
{
"_id" : "Asus",
"total" : 2,
"counts" : [
{
"rating" : "high",
"count" : 1
},
{
"rating" : "medium",
"count" : 1
}
]
}
or if you are after a more "flat" or "denormalised" result, run this aggregate operation:
db.mobile.aggregate([
{
"$group": {
"_id": "$manufacturer",
"total": { "$sum": 1 },
"high_ratings": {
"$sum": {
"$cond": [ { "$eq": [ "$rating", "high" ] }, 1, 0 ]
}
},
"medium_ratings": {
"$sum": {
"$cond": [ { "$eq": [ "$rating", "medium" ] }, 1, 0 ]
}
},
"low_ratings": {
"$sum": {
"$cond": [ { "$eq": [ "$rating", "low" ] }, 1, 0 ]
}
},
"none_ratings": {
"$sum": {
"$cond": [ { "$eq": [ "$rating", "none" ] }, 1, 0 ]
}
}
}
}
])
Sample Output
/* 1 */
{
"_id" : "Motorola",
"total" : 2,
"high_ratings" : 1,
"medium_ratings" : 0,
"low_ratings" : 0,
"none_ratings" : 1
}
/* 2 */
{
"_id" : "Asus",
"total" : 2,
"high_ratings" : 1,
"medium_ratings" : 1,
"low_ratings" : 0,
"none_ratings" : 0
}