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PONSFetchedResultsController and NSOrderedSet relationships
 

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    <p>I am having an issue (understanding issue to be honest) with NSFetchedResultsController and the new NSOrderedSet relationships available in iOS 5.</p> <p>I have the following data-model (ok, my real one is not drawer's and sock's!) but this serves as a simple example:</p> <p><img src="https://i.stack.imgur.com/dWE9y.png" alt="enter image description here"></p> <p>Drawer and Sock are both NSManagedObjects in a Core Data model/store. On <code>Drawer</code> the <code>socks</code> relationship is a <strong>ordered to-many</strong> relationship to <code>Sock</code>. The idea being that the socks are in the drawer in a specific order. On <code>Sock</code> the <code>drawer</code> relationship is the inverse of the <code>socks</code> relationship.</p> <p>In a UIViewController I am drawing a UITableView based on these entities. I am feeding the table using a <code>NSFetchedResultsController</code>. </p> <pre><code>- (NSFetchedResultsController *)fetchedResultsController1 { if (_fetchedResultsController1 != nil) { return _fetchedResultsController1; } NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Sock" inManagedObjectContext:[NSManagedObjectContext MR_defaultContext]]; [fetchRequest setEntity:entity]; NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"drawer.socks" ascending:YES]; [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]]; self.fetchedResultsController1 = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:[NSManagedObjectContext MR_defaultContext] sectionNameKeyPath:nil cacheName:@"SocksCache"]; self.fetchedResultsController1.delegate = self; return _fetchedResultsController1; } </code></pre> <p>When I run this, I get the following errror: <code>*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'to-many key not allowed here'</code></p> <p>This makes sense to me, as the relationship is a <code>NSOrderedSet</code> and not a single entity to compare against for sorting purposes.</p> <p>What I want to achieve is for the <code>Socks</code> to appear in the <code>UITableView</code> in the order specified in the <code>socks</code> relationship. I don't really want to have a sort order but <code>NSFetchedResultsController</code>, which is a great component is insisting there has to be one. How can I tell it to use the socks order on the Drawer entity. I don't want the table to show Drawer entities at all. </p> <p><strong>Note:</strong> I am using this within an iOS5 application only, so ordered relationships are available.</p> <p>Anyone that can offer me any direction, would be greatly appreciated. Thanks for your time.</p> <p><strong>Edit:</strong> So the tableview that display's socks does so for just one drawer. I just want the table view to honor the order that the socks relationship contains. I'm not sure what to set the sort criteria to make sure that happens.</p>
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    1. CODamien, you seem to have resolved this. When you say "in the order specified in the `socks` relationship" - how do you go about specifying the order of the relationship? In the editor I can just see a check-box (ordered or not ordered), but nothing to control the sort criteria.
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    2. CO@jhabbott. When you specify an ordered relationship (using checkbox), then the relationship is represented by a NSOrderedSet. You can then use the usual ordered set methods to control the order.
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    3. COOk, so now I start to use the generated `insertObject:inSocksAtIndex:` method, but I get an unrecognized selector exception. I thought the managed object context would add this at run-time?
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