Mongo Distinct Aggegation

Question

I\'m trying to use the aggregation framework to perform group counts in mongo but the results are not exactly as expected.

Consider the collection bellow

Answer 1

If I'm understanding the question right, what you're trying to get at is

 totals of all DISTINCT users_id under a day

Or as I understand it: Count of unique user_ids per day.

For that, you could take the group you already have and cut out the count so that you just have a unique _id.user_id and _id.day value:

'$group' => array(
            '_id'  => array(
                'user_id' => '$user_id',
                'day' => '$day'
            )
        )

Then pipe that to another $group statement that counts the number of documents per day, since there is exactly one for every unique user_id/day combination:

'$group' => array(
            '_id'  => '$_id.day',
            'count' => array('$sum' => 1)
        )