Instances of NSManagedObject
must be created using the designated initialiser
initWithEntity:insertIntoManagedObjectContext:
The Core Data property accessor methods are created dynamically at runtime, and that
cannot work if the object was created with a plain init
method.
This might work (untested):
AppointmentGrid *appointmentGridCopy = [[[self class] allocWithZone:zone]
initWithEntity:self.entity
insertIntoManagedObjectContext:self.managedObjectContext];