Parse.com: Sort by number of comments - ios

Question

I'm creating a UITableView with posts. If one of the tableview's rows is tapped, a detail page is shown where comments can be made to the post. I'm using the Parse.com framework to realize this.
I save the comments like this in the detailview:
PFObject *comment = [PFObject objectWithClassName:#"Comment"];
comment[#"content"] = _textViewComment.text;
comment[#"post"] = _object;
comment[#"user"] = currentUser;
[comment saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
if(!error){
UIAlertView* alertSave = [[UIAlertView alloc]initWithTitle:#"Saved" message:#"Your comment is saved" delegate:self cancelButtonTitle:#"OK" otherButtonTitles:nil];
[alertSave show];
[self setToDefault:_textViewComment];
}else{
UIAlertView *alert = [[UIAlertView alloc]initWithTitle:#"Warning" message:#"Comment not saved" delegate:self cancelButtonTitle:#"OK" otherButtonTitles:nil];
[alert show];
}
}];
where _object is the current post.
What I want to do now is sort the first tableview by number of comments. But I'm only making a relation from the comment to the post and not the other way around.
Is there a way to query the posts and sort them by number of comments?

Answer 1

To do this, you should relate comments to posts via an array on Post.
Add a "comments" col of type array on Post. When you fetch the post, use includeKey: on the #"comments" key. Then change your update code to add a comment like this...
// in your code, I think "_object" is a post. calling it "post" here instead...
NSMutableArray *comments = [[post objectForKey:#"comments"] mutableCopy];
[comments addObject:comment];
[post setObject:comments forKey:#comments"];
// then save the post object. parse will save the related comment.
[post saveInBackgroundWithBlock ...
Double check everything in the data browser. Then, after the next fetch of posts, you can sort by comment count...
[query findObjectsInBackgroundWithBlock:^(NSArray *array, NSError *error) {
if (!error) {
NSComparator comparator = ^(Post *postA, Post *postB) {
NSNumber *countA = #([postA objectForKey:#"comments"].count);
NSNumber *countB = #([postB objectForKey:#"comments"].count);
return [countA compare:countB];
};
NSArray *sortedArray = [array sortedArrayUsingComparator:comparator];
// use sortedArray as your tableview datasource
}
}];