sometimes when hacking one needs the actual int of the primary key. here is how one can grab it:
NSManagedObjectID *yourManagedObjectID = [yourManagedObject objectID];
int yourManagedObject_PK = [[[[[yourManagedObjectID URIRepresentation] absoluteString] lastPathComponent] substringFromIndex:1] intValue];
despite CoreData being an object graph, if one looks at the CoreData generated SQLite database data, this way of grabbing the primary key of an NSManagedObject should not be a problem. ii have used CoreData and the low level sqlite3 C library together in the same code and passing primary keys from CoreData to sqlite3 for fetching records works just fine.
! if You intend to use this code in production, be aware of possible internal changes to the way the db primary key transforms into a URIRepresentation, it might brake Your code.
enjoy