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POHow to define a NSPredicate which specifies a having max(x) clause using group by?
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  1. POHow to define a NSPredicate which specifies a having max(x) clause using group by?
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    copied!<p>I am having trouble creating a valid NSPredicate within an iOS-CoreData context. The table contains all messages of all buddies. I need to get the newest message of each buddy:</p> <p>I have tried using DBVisualizer and I need the following SQL (which works fine) </p> <pre><code> /* for testing with a db tool like DbVisualizer select ZBAREJIDSTR,ZBODY, ZTIMESTAMP from ZXMPPMESSAGEARCHIVING_MESSAGE_COREDATAOBJECT group by ZBAREJIDSTR having max(ZTIMESTAMP) order by ZTIMESTAMP desc; */ </code></pre> <p>The following code fails with "Unable to parse the format string" and I could not find an example which worked for me. </p> <pre><code> - (NSFetchedResultsController *)fetchedResultsController { if (fetchedResultsController == nil) { NSManagedObjectContext *moc = [[Utilities chatManagerFromAppDelegate] managedObjectContext_messageArchiving]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"XMPPMessageArchiving_Message_CoreDataObject" inManagedObjectContext:moc]; NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; NSAttributeDescription* bodyDesc = [entity.attributesByName objectForKey:@"body"]; NSAttributeDescription* barJidStrDesc = [entity.attributesByName objectForKey:@"barJidStr"]; NSAttributeDescription* timestampDesc = [entity.attributesByName objectForKey:@"timestamp"]; NSPredicate *predicateMaxTimestamp = [NSPredicate predicateWithFormat:@"@max.timestamp"]; NSSortDescriptor *sortDescriptorTimestampDesc = [[NSSortDescriptor alloc] initWithKey:@"timestamp" ascending:NO]; NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptorTimestampDesc, nil]; [fetchRequest setEntity:entity]; [fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:barJidStrDesc, bodyDesc, timestampDesc, nil]]; [fetchRequest setPropertiesToGroupBy:[NSArray arrayWithObject:bodyDesc]]; [fetchRequest setHavingPredicate: predicateMaxTimestamp]; [fetchRequest setSortDescriptors:sortDescriptors]; [fetchRequest setResultType:NSDictionaryResultType]; [fetchRequest setFetchBatchSize:10]; fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:moc sectionNameKeyPath:nil cacheName:nil]; .... </code></pre> <p>Actually I would also appreciate other alternatives, because I need updates of the underlaying model ([fetchedResultsController setDelegate:self];) which is not working with a NSDictionaryResultType (but NSDictionaryResultType is necessary for with a group by).</p>
 

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