解决方法
只需在createdAt上添加降序并获取第一个对象：
PFQuery *query = [PFQuery queryWithClassName:@"YourClassName"];
[query orderByDescending:@"createdAt"];
[query getFirstObjectInBackgroundWithBlock:^(PFObject *object,NSError *error) {
// code
}];
在斯威夫特：
var query = PFQuery(className: "YourClassName")
query.orderByDescending("createdAt")
query.getFirstObjectInBackgroundWithBlock {(object: PFObject?,error: NSError?) -> Void in
// code
}
