[{"_id":1,"entities":{"hashtags":[{"indices":[124,136],"text":"MyTag"}]},"user":{"id":456,"id_str":"456","name":"Author Name","screen_name":"Author Screen Name"}},{"_id":ObjectId("604c805b289d1ef5947e1845"),"entities":{"hashtags":[{"indices":[124,136],"text":"MyTAG"}],}}]

db.collection.aggregate([{"$project":{"tags":"$entities.hashtags.text","_id":0}},{"$unwind":"$tags"},{"$group":{"_id":{$toLower:"$tags"},"count":{"$sum":1}}}])