Parse - 如何设置多个父对象

Question

对于我的应用程序，我希望上传的图像有2个父对象，因为根据我自己的理解，这是接近它的最佳方式之一 - 但如果我错了，请纠正我。以下是我正在尝试的代码。

// Create a PFObject around a PFFile and associate it with the current user
PFObject *userPhoto = [PFObject objectWithClassName:@"UserPhoto"];
[userPhoto setObject:imageFile forKey:@"imageFile"];

PFUser *user = [PFUser currentUser];
[userPhoto setObject:user forKey:@"user"];

// Create PFObject and associate userPhoto with Listings as well
PFObject *listings = [PFObject objectWithClassName:@"Listings"];
userPhoto[@"parent"] = listings;

[userPhoto saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
    if (!error) {
        [self refresh:nil];
    }
    else{
        // Log details of the failure
        NSLog(@"Error: %@ %@", error, [error userInfo]);
    }
}];

它似乎不起作用。任何见解都会有所帮助