List days occuring in dataset - mongodb

Question

I have this huge dataset for which every entry has a datetime field. The data was inserted irregularly. For example:
2015-04-20 : 500 entries,
2015-04-23 : 300 entries,
2015-05-01 : 600 entries
The thing is, I do not know when these active days are. What I would like is a mongodb query which returns some sort of array containing all days which occur in the database, like so:
['2015-04-20,
'2015-04-23,
'2015-04-23,
'2015-04-25,
'2015-05-01,
'2015-05-05,
'2015-05-09]
Is this possible, and if so: how can I achieve this?

Answer 1

There is a "distinct" command that has shell wrapper, which can be used something like:
db.collection.distinct(dateFieldName, query)
If you are not running it from shell, check whether your driver wraps this command, if not you can use the command directly:
{ distinct: "<collection>", key: "<field>", query: <query> }
http://docs.mongodb.org/manual/reference/command/distinct/#dbcmd.distinct
If your time stamp field needs some additinal processing, you can use aggregation framework.
db.collection.aggregate([{$group: {_id: $substr: ["$timestamp", 0, 10]}}]
http://docs.mongodb.org/v2.6/core/aggregation-introduction/

Answer 2

Assuming a field named dateField that contains Date values, you can use the aggregation date operators with $group to do this.
It's easiest if you're using Mongo 3.x where the $dateToString operator is available:
db.dates.aggregate([
{$group: {
_id: {$dateToString: {format: '%Y-%m-%d', date: '$dateField'}},
count: {$sum: 1}
}},
{$sort: {count: -1}}
])
Prior to 3.0 you need to use multiple date operators to piece together the date into the _id when grouping:
db.dates.aggregate([
{$group: {
_id: {
year: {$year: '$dateField'},
month: {$month: '$dateField'},
day: {$dayOfMonth: '$dateField'}
},
count: {$sum: 1}
}},
{$sort: {count: -1}}
])
In both cases, note the use of $sort to order the results by the number of docs on each day, descending.