[{"key":1,"upvote":[1,3,5,3]},{"key":2,"upvote":[1,5,3]},{"key":3,"upvote":[1,3]},{"key":4}]

db.collection.aggregate([{$project:{upvote_count:{$size:{"$ifNull":["$upvote",[]]}},key:1,upvote:1}},{$sort:{"upvote_count":-1}}])