[{_id:ObjectId("60e80c96b9c55e7a01898f5c"),"watched":{"1":true,"2":true,"3":true},"fans":4}]

db.collection.update({_id:ObjectId("60e80c96b9c55e7a01898f5c")},[{"$set":{"fans":{"$add":["$fans",{"$cond":{"if":{"$ne":["$watched.3",undefined]},"then":1,"else":-2}}]}}},{"$unset":"watched.3"}])