[{"title":"title1","author":[{"name":"user1","register":true},{"name":"user2","register":true}],"tags":["tag1","tag2","tag3"]},{"title":"title2","author":[{"name":"user1","register":true},{"name":"user2","register":true}],"tags":["tag1","tag2","tag3"]},{"title":"title3","author":[{"name":"user1","register":true},{"name":"user2","register":true}],"tags":["tag1","tag2","tag3"]}]

db.collection.aggregate([{"$unwind":"$tags"},{"$group":{"_id":"$tags","titles":{"$push":"$title"}}},{$project:{tag:"$tag",titles:1,size:{$size:"$titles"},_id:"$$REMOVE"}}])