[{"_id":"1","title":"Harry Potter","year":2000,"author":"JK. Rowling","genres":["Fantasy","War"]},{"_id":"2","title":"Harry Potter 99","year":2020,"author":"JK. Rowling","genres":"Drama"},{"_id":"3","title":"Harry Potter","year":2000,"author":"JK. Rowling","genres":["Drama","Crime"]},]

db.collection.aggregate([{"$match":{"author":"JK. Rowling"}},{"$unwind":"$genres"},{"$group":{"_id":"$author","genres":{"$addToSet":"$genres"}}},{"$project":{"numgenres":{"$size":"$genres"},"genres":1}},{"$sort":{"numgenres":-1}}])