Traversing multiple Core Data objects using NSPredicate

Question

I am having a problem with a Core Data model in Cocoa. It's probably a fairly basic problem. A section of my Core Data model is shown below. Given the value of a cell property in the OutputCell entity, I want to return the relevant HistogramBar.

I'm using the following Predicate but it just returns an empty array. I've managed to get it working using the Histogram entity but I don't seem to be able to traverse from HistogramBar through Histogram and on to OuputCell. The predicate I'm using is:

NSEntityDescription *histogramBarEntityDescription = [NSEntityDescription entityForName:@"HistogramBar" 
                                                                inManagedObjectContext:[theDocument managedObjectContext]];
NSFetchRequest *histogramBarRequest = [[[NSFetchRequest alloc] init] autorelease];
[histogramBarRequest setEntity:histogramBarEntityDescription];
NSPredicate *histogramBarPredicate = [NSPredicate predicateWithFormat:@"(histogram.outputCell.cell = %@)", theOutputCell];
[histogramBarRequest setPredicate:histogramBarPredicate];
NSError *histogramBarError = nil;
NSArray *histogramsArray = [[theDocument managedObjectContext] executeFetchRequest:histogramBarRequest 
     error:&histogramBarError];

Thankyou for the help.

Can you post a little more code? I'd like to see the entire fetch request. — sbooth
– sbooth, Commented Mar 2, 2009 at 15:54

Ian Turner · Accepted Answer · 2009-03-02 22:50:13Z

2

My problem is solved, a rogue comment in the code prevented the HistogramBar entities being created. In this instance detailed checking of some NSLog's helped spot the problem.

answered Mar 2, 2009 at 22:50

Ian Turner

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