[{"id":123,"source":"abc","media":[{"index_num":0,"media_url":"some url"},{"index_num":1,"media_url":"some url"},{"index_num":2,"media_url":"some url"}]}]

db.collection.aggregate([{$set:{"new_media":{"media_info":"$media","num_images":{$size:"$media"}}}},{$set:{media:"$new_media"}},{$unset:"new_media"}])