db={"hashtags":[{"_id":"1","hashtag":["hashtag1","hashtag2"]},{"_id":"2","hashtag":["hashtag1","hashtag2"]},{"_id":"3","hashtag":["hashtag1"]}]}

db.hashtags.aggregate([{"$unwind":"$hashtag"},{"$group":{"_id":"$hashtag","count":{"$sum":1}}},{"$project":{"_id":0,"hashtag":"$_id","count":1}}])