> Hi Kirill,

> Check out the $unwind operator
> <http://docs.mongodb.org/manual/reference/aggregation/#_S_unwind>which
> will take the documents in your aggregation pipeline and multiplexes the
> array entries as separate documents, eg.

> Before $unwind:

> {
> a: 1,
> b: [ 2,3,4]
> }

> After $unwind:

> { a:1,b:2 },
> { a:1,b:3 },
> { a:1,b:4 }

> After you include the $unwind in your pipeline, add the $group operation
> on "$events.name".

> -Stephen

> On Tuesday, November 13, 2012 4:13:35 PM UTC-5, kirill kazachenko wrote:

>> Hello, i have a question about grouping by sub elements.

>> i have next structure:
>> {
>> "name":"name",
>> "events": [
>>      {"name":"event1", "date": ...},
>>      {"name":"event2", "date": ...}
>>   ]
>> }

>> I'm need to extract and group all events by name with it's count, how can
>> i do this with the aggregation?

>> db.sessions.aggregate([{$group: {_id:"$events.name"}}])

>> i have response:

>> {
>>         "result" : [
>>                 {
>>                         "_id" : [ ]
>>                 },
>>                 {
>>                         "_id" : [
>>                                 "open_home",
>>                                 "open_home",
>>                                 "open_homdfe",
>>                                 "open_homdfe",
>>                                 "4en_homdfe",
>>                                 "4en_homdfe"
>>                         ]
>>                 }
>>         ],
>>         "ok" : 1
>> }