Add support for $count aggregation stage

[evanchooly]

JAVA-2277

[email2liyang]

@evanchooly any update on this? thanks

[evanchooly]

I left the company some months back and no longer have write access to this repository. i can fork and submit a PR but I just haven't had the time.

[email2liyang]

@evanchooly thanks, is there any other repo maintainer for morphia project?

[jyemin]

The $count stage is just syntactic sugar for:

{$group:{_id:null, count:{$sum:1}}}

so you should be able to accomplish this with the existing aggregation support in Morphia. See http://mongodb.github.io/morphia/1.3/guides/aggregation/ to get you started.

[email2liyang]

it works well, thanks!