Parse.com orderByDescending不起作用

Question

我有以下代码：

PFQuery *innerQuery = [PFQuery queryWithClassName:@"Post"];
[innerQuery whereKeyExists:@"image"];
[innerQuery orderByDescending:@"createdAt"];

PFQuery *query = [PFQuery queryWithClassName:@"Comment"];
[query whereKey:@"post" doesNotMatchQuery:innerQuery];
[query findObjectsInBackgroundWithBlock:^(NSArray *comments, NSError *error) {
    // comments now contains the comments for posts without images
}];

但是orderby不起作用。有没有办法解决这个问题？感谢