MongoDB count distinct items - mongodb

Question

I have following query on a list with this fields : key,time,p,email
use app_db;
db.getCollection("app_log").aggregate(
[
{
"$match" : {
"key" : "login"
}
},
{
"$group" : {
"_id" : {
"$substr" : [
"$time",
0.0,
10.0
]
},
"total" : {
"$sum" : "$p"
},
"count" : {
"$sum" : 1.0
}
}
}
]
);
and the output is something like this :
{
"_id" : "2019-08-25",
"total" : NumberInt(623),
"count" : 400.0
}
{
"_id" : "2019-08-24",
"total" : NumberInt(2195),
"count" : 1963.0
}
{
"_id" : "2019-08-23",
"total" : NumberInt(1294),
"count" : 1706.0
}
{
"_id" : "2019-08-22",
"total" : NumberInt(53),
"count" : 1302.0
}
But I need the count to be distinctive on email field, which is count number of distinct email addresses who logged in per day and their p value is greater 0

Answer 1

You need $addToSet to get an array of unique email values per day and then you can use $size to count the number of items in that array:
db.getCollection("app_log").aggregate(
[
{
"$match" : {
"key" : "login"
}
},
{
"$group" : {
"_id" : {
"$substr" : [
"$time",
0.0,
10.0
]
},
"total" : {
"$sum" : "$p"
},
"emails" : {
"$addToSet": "$email"
}
}
},
{
$project: {
_id: 1,
total: 1,
countDistinct: { $size: "$emails" }
}
}
]
);