Core data, reacting to a change in the interface

when using core data there is a text field that sets a value for an entity's string property.

when this property is changed i want a whole bunch of things to change when depending on what that string was.

what i did was (what i think) override the setPropertyOne and getPropertyOne methods (where propertyOne is the string property of the class)

using the core data utility tutorial as a guide i typed something like this:

Code:

-(NSString*)propertyOne {
	[self willAccessValueForKey: @"propertyOne"];
	NSString* s = propertyOne;
	[self didAccessValueForKey: @"propertyOne"];
	return s;
}
-(void)setPropertyOne: (NSString*)newPropertyOne {
	
	[self willChangeValueForKey: @"propertyOne"];
	propertyOne = newPropertyOne;
	[self didChangeValueForKey: @"propertyOne"];

now this all works fine... other than the point where i undo and redo
undo works fine, changing propertyOne back to the prior string works... redo breaks
it breaks with this being printed out to the console:

also when i added code to make new entityTwo instances and add them to the set that carries those instances undo would work fine, but if i did that then the program would crash without saying anything when i went to add a new instance of entityOne