Mongo Aggregation : $group and $project array to object for counts - mongodb

Question

I have documents like:
{
"platform":"android",
"install_date":20151029
}
platform - can have one value from [android|ios|kindle|facebook ] .
install_date - there are many install_dates
There are also many fields.
Aim : I am calculating installs per platform on particular date.
So I am using group by in aggregation framework and make counts by platform. Document should look like like:
{
"install_date":20151029,
"platform" : {
"android":1000,
"ios": 2000,
"facebook":1500
}
}
I have done like:
db.collection.aggregate([
{
$group: {
_id: { platform: "$platform",install_date:"$install_date"},
count: { "$sum": 1 }
}
},
{
$group: {
_id: { install_date:"$_id.install_date"},
platform: { $push : {platform :"$_id.platform", count:"$count" } }
}
},
{
$project : { _id: 0, install_date: "$_id.install_date", platform: 1 }
}
])
which Gives document like:
{
"platform": [
{
"platform": "facebook",
"count": 1500
},
{
"platform": "ios",
"count": 2000
},
{
"platform": "android",
"count": 1000
}
],
"install_date": 20151027
}
Problem:
Projecting array to single object as "platform"

Answer 1

With MongoDb 3.4 and newer, you can leverage the use of $arrayToObject operator to get the desired result. You would need to run the following aggregate pipeline:
db.collection.aggregate([
{ "$group": {
"_id": {
"date": "$install_date",
"platform": { "$toLower": "$platform" }
},
"count": { "$sum": 1 }
} },
{ "$group": {
"_id": "$_id.date",
"counts": {
"$push": {
"k": "$_id.platform",
"v": "$count"
}
}
} },
{ "$addFields": {
"install_date": "$_id",
"platform": { "$arrayToObject": "$counts" }
} },
{ "$project": { "counts": 0, "_id": 0 } }
])
For older versions, take advantage of the $cond operator in the $group pipeline step to evaluate the counts based on the platform field value, something like the following:
db.collection.aggregate([
{ "$group": {
"_id": "$install_date",
"android_count": {
"$sum": {
"$cond": [ { "$eq": [ "$platform", "android" ] }, 1, 0 ]
}
},
"ios_count": {
"$sum": {
"$cond": [ { "$eq": [ "$platform", "ios" ] }, 1, 0 ]
}
},
"facebook_count": {
"$sum": {
"$cond": [ { "$eq": [ "$platform", "facebook" ] }, 1, 0 ]
}
},
"kindle_count": {
"$sum": {
"$cond": [ { "$eq": [ "$platform", "kindle" ] }, 1, 0 ]
}
}
} },
{ "$project": {
"_id": 0, "install_date": "$_id",
"platform": {
"android": "$android_count",
"ios": "$ios_count",
"facebook": "$facebook_count",
"kindle": "$kindle_count"
}
} }
])
In the above, $cond takes a logical condition as it's first argument (if) and then returns the second argument where the evaluation is true (then) or the third argument where false (else). This makes true/false returns into 1 and 0 to feed to $sum respectively.
So for example, if { "$eq": [ "$platform", "facebook" ] }, is true then the expression will evaluate to { $sum: 1 } else it will be { $sum: 0 }