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

db.collection.update({_id:ObjectId("60e80c96b9c55e7a01898f5c"),"watched.3":{$exists:true}},{$inc:{fans:-1}})