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SQL and XML stores behave differently with sort descriptor

Subject: SQL and XML stores behave differently with sort descriptor
From: Scott Ellsworth <email@hidden>
Date: Thu, 11 Aug 2005 13:55:04 -0700
Delivered-to: email@hidden
Delivered-to: email@hidden

Hi, all.

I am getting an error with a SQL store that I am not with an XML store, and I want to know whether the behavioral difference is a bug worth filing.

I have a custom subclass of an entity in my Core Data object model. It has a method that returns a computed property. That property is not a part of the object model, but adding it as a transient attribute did not make a difference. I wanted to do a fetch, ordered by this attribute.

The implementation:

- (float)tradeBetweenAlongRoute{ return fmin([[[self destinationWorld] tradeRating] floatValue], [self tradeMod]); }

I then do a fetch, ordered by this attribute:

- (NSArray *)biggestTradePartnersForWorld:(World *)world { NSDictionary * dict = [NSDictionary dictionaryWithObject:world forKey:@"WORLD"]; NSFetchRequest * fetchRequest = [[self managedObjectModel] fetchRequestFromTemplateWithName:@"tradeRelationshipsFromWorld" substitutionVariables:dict];

NSSortDescriptor * tradeSorter=[[[NSSortDescriptor alloc] initWithKey:@"tradeBetweenAlongRoute" ascending:NO] autorelease]; NSArray * sortDescriptors=[NSArray arrayWithObject:tradeSorter]; [fetchRequest setSortDescriptors:sortDescriptors];

NSError * error = nil; NSArray * tradeRelationships = [[self managedObjectContext] executeFetchRequest:fetchRequest error:&error]; if (tradeRelationships == nil) NSLog(@"Error: %@", error);

    return tradeRelationships;
}

If I load a data file into a brand new XML store, this gives me a list of results.

If I saveAs a SQL store, it throws the error (while executing the fetch request):

2005-08-11 13:21:34.610 Astrogator[11881] unresolved keypath: tradeBetweenAlongRoute

The reason is clear - CD does not have the needed information to do the orderBy on the server. The workaround was to do the sort post fetch. The following works:

- (NSArray *)biggestTradePartnersForWorld:(World *)world { NSDictionary * dict = [NSDictionary dictionaryWithObject:world forKey:@"WORLD"]; NSFetchRequest * fetchRequest = [[self managedObjectModel] fetchRequestFromTemplateWithName:@"tradeRelationshipsFromWorld" substitutionVariables:dict];

NSError * error = nil; NSArray * tradeRelationships = [[self managedObjectContext] executeFetchRequest:fetchRequest error:&error]; if (tradeRelationships == nil) NSLog(@"Error: %@", error);

NSSortDescriptor * tradeSorter=[[[NSSortDescriptor alloc] initWithKey:@"tradeBetweenAlongRoute" ascending:NO] autorelease]; NSArray * sortDescriptors=[NSArray arrayWithObject:tradeSorter]; tradeRelationships = [tradeRelationships sortedArrayUsingDescriptors:sortDescriptors];

    return tradeRelationships;
}

I would have expected the code to fail in the same way for both stores. (I ended up surprised when I did a saveAs and the broken code showed that it was broken.)

Is this worth filing?

Scott
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