I have two entities specified in XCode{A, B}. A has a 1 to 1 relationship with B, while B has 1 to many. Now B(totalDistance) has a property which is dependent on the sum of all the properties in each A(distance). Incidentally the Core Data Programming Guide: Core Data FAQ section of the documentation has a section which describes my problem with a solution. So I went ahead and implemented the solution. I added the method the following methods to B

• (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {

  if([keyPath isEqual:@”distance”]) { NSLog(@”receiving notification that the distance changed”); [self updateTotalDistance]; } }

• (void)updateTotalDistance { NSNumber *totalDistance = [self valueForKeyPath:@”email@hiddennce”]; [self setTotalDistance: [totalDistance floatValue]]; }

Now in the A entity I have the following:

• (void)setB:(NSManagedObject *)value { [self willChangeValueForKey: @”b”]; [self setPrimitiveValue: value forKey: @”b”]; [self didChangeValueForKey: @”b”];

  [self addObserver:[self b] forKeyPath:@”distance” options:(NSKeyValueObservingOptionNew) context:NULL];

}

So everything compiles just fine but when I run and I change the distance in A I get the following error:

-observeValueForKeyPath:ofObject:change:context: only defined for abstract class. Define -[NSManagedObject observeValueForKeyPath:ofObject:change:context:]!

Any ideas? I usually try as hard to find a solution before posting but I have been stuck on this one for a couple of days.

Thank You.

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I might be wrong, but I think you actually want to be using dependent keys. See the KVO method + (void)setKeys:(NSArray *)keys triggerChangeNotificationsForDependentKey:(NSString *)dependentKey covered here: file:///Developer/ADC%20Reference%20Library/documentation/Cocoa/Conceptual/KeyValueObserving/index.html

You would define dependent keys in your object’s +initialize method.

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According to the Core Data FAQ that will NOT work. Apparently I have to use what I implemented. What I a don’t understand is why am am not receiving the message in the entity.