Crash in OS X Core Data Utility Tutorial - objective-c

Question

I'm trying to follow Apple's Core Data utility Tutorial. It was all going nicely, until...
The tutorial uses a custom sub-class of NSManagedObject, called 'Run'. Run.h looks like this:
#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>
#interface Run : NSManagedObject {
NSInteger processID;
}
#property (retain) NSDate *date;
#property (retain) NSDate *primitiveDate;
#property NSInteger processID;
#end
Now, in Run.m we have an accessor method for the processID variable:
- (void)setProcessID:(int)newProcessID {
[self willChangeValueForKey:#"processID"];
processID = newProcessID;
[self didChangeValueForKey:#"processID"];
}
In main.m, we use functions to set up a managed object model and context, instantiate an entity called run, and add it to the context. We then get the current NSprocessInfo, in preparation for setting the processID of the run object.
NSManagedObjectContext *moc = managedObjectContext();
NSEntityDescription *runEntity = [[mom entitiesByName] objectForKey:#"Run"];
Run *run = [[Run alloc] initWithEntity:runEntity insertIntoManagedObjectContext:moc];
NSProcessInfo *processInfo = [NSProcessInfo processInfo];
Next, we try to call the accessor method defined in Run.m to set the value of processID:
[run setProcessID:[processInfo processIdentifier]];
And that's where it's crashing. The object run seems to exist (I can see it in the debugger), so I don't think I'm messaging nil; on the other hand, it doesn't look like the setProcessID: message is actually being received. I'm obviously still learning this stuff (that's what tutorials are for, right?), and I'm probably doing something really stupid. However, any help or suggestions would be gratefully received!
===MORE INFORMATION===
Following up on Jeremy's suggestions:
The processID attribute in the model is set up like this:
NSAttributeDescription *idAttribute = [[NSAttributeDescription alloc]init];
[idAttribute setName:#"processID"];
[idAttribute setAttributeType:NSInteger32AttributeType];
[idAttribute setOptional:NO];
[idAttribute setDefaultValue:[NSNumber numberWithInteger:-1]];
which seems a little odd; we are defining it as a scalar type, and then giving it an NSNumber object as its default value. In the associated class, Run, processID is defined as an NSInteger. Still, this should be OK - it's all copied directly from the tutorial.
It seems to me that the problem is probably in there somewhere. By the way, the getter method for processID is defined like this:
- (int)processID {
[self willAccessValueForKey:#"processID"];
NSInteger pid = processID;
[self didAccessValueForKey:#"processID"];
return pid;
}
and this method works fine; it accesses and unpacks the default int value of processID (-1).
Thanks for the help so far!

Answer 1

If you are getting EXC_BAD_ACCESS on
[run setProcessID:[processInfo processIdentifier]];
it's almost certainly due to one of the pointers no longer pointing to a real object. Either run has been dealloc'd or processInfo has been dealloc'd. This assumes that the line is not the next line of code after
NSProcessInfo *processInfo = [NSProcessInfo processInfo];
If it is, then both objects should be valid, so you are probably looking at something wrong with
[self willChangeValueForKey:#"processID"];
or
[self didChangeValueForKey:#"processID"];
if you have any objects observing that key, it's possible they have gone stale somehow.

Answer 2

This did not solve my problem with willAccessValueForKey but it solved a mystery. I subclassed an entity but I forgot to set the custom class in the model. The custom class worked until I wrote a method to send back a string which was a concatenation of some of the properties. The method was nearly identical to another method that was in another custom NSManagedObject class. I could not figure out why the one class worked and the other didn't. Once I set the custom class in the model they both worked.