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Xcode4 Core Data Relationships
// test/learn the core data frame work
NSManagedObjectContext *context = [self managedObjectContext];
NSManagedObject *locationData = [NSEntityDescription
                                   insertNewObjectForEntityForName:@"LocationData"
                                   inManagedObjectContext:context];
[locationData setValue:@"Testville" forKey:@"City"];
[locationData setValue:@"United Tests" forKey:@"Country"];
[locationData setValue:@"County of Test" forKey:@"County"];
NSManagedObject *zipCodeData = [NSEntityDescription
                                      insertNewObjectForEntityForName:@"ZipCodeData"
                                      inManagedObjectContext:context];
[zipCodeData setValue:[NSNumber numberWithDouble:1111.00] forKey:@"Income"];
[zipCodeData setValue:[NSNumber numberWithDouble:22.00] forKey:@"LandArea"];
[zipCodeData setValue:@"23060" forKey:@"ZipCode"];
NSError *error;
if (![context save:&error]) {
    NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
}

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
                               entityForName:@"CrossReference" inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
for (NSManagedObject *info in fetchedObjects) {
    NSLog(@"LocationId: %@", [info valueForKey:@"LocationDataId"]);
    NSManagedObject *details = [info valueForKey:@"details"];
    NSLog(@"ZipId: %@", [details valueForKey:@"ZipCodeDataId"]);
}
[fetchRequest release];

NSManagedObject *crossReference = [NSEntityDescription
                                   insertNewObjectForEntityForName:@"CrossReference"
                                   inManagedObjectContext:context];
  [crossReference setValue:locationData forKey:@"location"];
  [crossReference setValue:zipCodeData forKey:@"zipCode"];