Count Distinct Within Date Range - mongodb

Question

I have a MongoDB database with a collection of site-events. The documents look like:
{
"_id" : ObjectId("5785bb02eac0636f1dc07023"),
"referrer" : "https://example.com",
"_t" : ISODate("2016-07-12T18:10:17Z"),
"_p" : "ucd7+hvjpacuhtgbq1caps4rqepvwzuoxm=",
"_n" : "visited site",
"km screen resolution" : "1680x1050"
},
{
"_id" : ObjectId("5785bb02eac0636f1dc07047"),
"url" : "https://www.example.com/",
"referrer" : "Direct",
"_t" : ISODate("2016-07-12T18:10:49Z"),
"_p" : "txt6t1siuingcgo483aabmses2et5uqk0=",
"_n" : "visited site",
"km screen resolution" : "1366x768"
},
{
"_id" : ObjectId("5785bb02eac0636f1dc07053"),
"url" : "https://www.example.com/",
"referrer" : "Direct",
"_t" : ISODate("2016-07-12T18:10:56Z"),
"_p" : "gcama1az5jxa74wa6o9r4v/3k+zulciqiu=",
"_n" : "visited site",
"km screen resolution" : "1366x768"
}
I want to get a count of the unique persons within a date range. In SQL it would be
SELECT COUNT(DISTINCT(`_p`)) FROM collection WHERE `_t` > '<SOME DATE>' AND `_t` <= '<SOME OTHER DATE>'
So far, I've grouped the dates along using the aggregation pipeline:
db.siteEvents.aggregate(
[
{
$match : {"_n": "visited site"}
},
{
$group : {
_id: {
year : { $year : "$_t" },
month : { $month : "$_t" },
day : { $dayOfMonth : "$_t" },
_p : "$_p"
},
count: { $sum: 1 }
}
},
{
$group : {
_id : {
year : { $year : "$_id.year" },
month : { $month : "$_id.month" },
day : { $dayOfMonth : "$_id.day" }
},
count: { $sum: 1 }
}
}
]
);
But this gives errors - I believe because of the second grouping _id trying to grab an intermediate field. I'm currently just using the Mongo shell, but if I had to choose an alternative driver it would be PyMongo. I'd like to get this to work in the shell (so I can understand the process).

Answer 1

With an aggregation pipeline it could look like so
db.getCollection('siteEvents').aggregate([
{
$match: {
_t: {
$gt: ISODate("2016-07-11T08:10:17.000Z"),
$lt: ISODate("2016-07-12T14:10:17.000Z")
}
}
},
{
$group: {
_id: "$_p"
}
},
{
$group: {
_id: null,
distinctCount: { $sum: 1 }
}
}
])
If you know the resulting distinct values won't be large then you could use a simply query like so
db.getCollection('siteEvents').distinct(
'_p',
{
_t: {
$gt: ISODate("2016-07-11T08:10:17.000Z"),
$lt: ISODate("2016-07-12T14:10:17.000Z")
}
}).length

Answer 2

You can use the $addToSet operator in the $group stage to return an array of distinct "_p" value then $project the resulted document to return the size of the array which is nothing other than the distinct count.
db.siteEvents.aggregate(
[
{"$match": {"_n": "visited site", "_t": {"$gt": <SOME DATE>, "$lt": <SOME OTHER DATE>}}},
{"$group": {
"_id": None,
"_p_values": {"$addToSet": "$_p"}
}},
{"$project": {"_id": 0, "count": {"$size": "$_p_values"}}}
]
)
For small size collection you can simply use distinct but you need to pass in the query argument.
len(db.siteEvents.distinct("_p", {"_n": "visited site", "_t": {"$gt": <SOME DATE>, "$lt": <SOME OTHER DATE>}}))