Calculate distinct count on fields in mongodb - mongodb

Question

I have following collection:
[{
"id": 1,
"activity_type": "view",
"user_id": 1
},
{
"id": 2,
"activity_type": "save",
"user_id": 1
},
{
"id": 3,
"activity_type": "save",
"user_id": 1
},
{
"id": 4,
"activity_type": "save",
"user_id": 2
}]
I need to get a result like this:
[{
"activity_type": "view",
"count": 1,
"user_count": 1
},{
"activity_type": "save",
"count": 3,
"user_count": 2
}]
So far I reached on this:
db.getCollection('activities').aggregate([
{
$group:{_id:"$activity_type", count: {$sum: 1}}
},
{
$project:
{
_id: 0,
activity_type: "$_id",
count: 1
}
}
])
It gives me:
[{
"activity_type": "view",
"count": 1
},
{
"activity_type": "save",
"count": 3
}]
How can I add distinct user_id count as well?

Answer 1

What you need to do is use $addToSet in the group stage to gather the unique ids, after that in the $project stage you can use $size to show the proper user count.
db.collection.aggregate([
{
$group: {
_id: "$activity_type",
count: {
$sum: 1
},
user_ids: {
"$addToSet": "$user_id"
}
}
},
{
$project: {
_id: 0,
activity_type: "$_id",
user_count: {
$size: "$user_ids"
},
count: 1
}
}
])
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