Group and sum day by day - mongodb

Question

This is how my collection structure looks like:
{
"_id" : ObjectId("57589d2a9108dace306602b8"),
"IDproject" : NumberLong(53),
"email" : "john.doe#gmail.com",
"dc" : ISODate("2016-06-06T22:33:13.000Z")
}
{
"_id" : ObjectId("57589d2a9108dace306602b8"),
"IDproject" : NumberLong(53),
"email" : "david.doe#gmail.com",
"dc" : ISODate("2016-06-07T22:33:13.000Z")
}
{
"_id" : ObjectId("57589d2a9108dace306602b8"),
"IDproject" : NumberLong(53),
"email" : "elizabeth.doe#gmail.com",
"dc" : ISODate("2016-06-078T22:33:13.000Z")
}
As you can see, there are two customers added on June 7th and one on June 6th. I would like to group and sum these results for the last 30 days.
It should looks something like this:
{
"dc" : "2016-06-05"
"total" : 0
}
{
"dc" : "2016-06-06"
"total" : 1
}
{
"dc" : "2016-06-07"
"total" : 2
}
As, you can see, there are no records on June 6th, so it's zero. It should be zero for June 5th, etc.
That would be the case #1, and the case #2 are following results:
{
"dc" : "2016-06-05"
"total" : 0
}
{
"dc" : "2016-06-06"
"total" : 1
}
{
"dc" : "2016-06-07"
"total" : 3
}
I've tried this:
db.getCollection('customer').aggregate([
{$match : { IDproject : 53}},
{ $group: { _id: "$dc", total: { $sum: "$dc" } } }, ]);
But seems complicated. I'm first time working with noSQL database.
Thanks.

Answer 1

Here's how you will get daily counts (the common idiom for row count is {$sum: 1}).
However, you cannot obtain zeros for days that are lacking data – because there is no data that would give the grouping key for these days. You must handle these cases in PHP by generating a list of desided dates and then looking if there's data for that each date.
db.getCollection('customer').aggregate([
{$match : { IDproject : 53}},
{$group: {
_id: {year: {$year: "$dc"}, month: {$month: "$dc"}, day: {$dayOfMonth: "$dc"}}},
total: {$sum: 1}
}},
]);
Note that MongoDB only operates in the UTC timezone; there are no aggregation pipeline operators that can convert timestamps to local timezones reliably. The $year, $month and $dayOfMonth operators give the date in UTC which may not be the same as in the local timezone. Solutions include:
saving timestamps in the local timezone (= lying to MongoDB that they are in UTC),
saving the timezone offset with the timestamp,
saving the local year, month and dayOfMonth with the timestamp.