[{_id:"1",quote:"live every day like it's your last",likes:["userId2","userId3"]}]

db.collection.update({"_id":"1"},[{$set:{likes:{$cond:[{$setIsSubset:[["userId"],"$likes"]},{$setDifference:["$likes",["userId"]]},{$concatArrays:["$likes",["userId"]]},]}}}])