Why does Core Data create NSManagedObject's with properties but no accompanying instance variable?

Question

Exactly as the title says, why does Core Data create instances of NSManagedObject with properties for each entity's attribute, but no accompanying instance variable? The problem is, I would like to use some of my 'Entities' in a typical alloc/init style fashion in some parts of my code. Not using Core Data's fetching/context to create/store.

I suppose I could stay without the instance variables, but would it hurt to add them? Can I also change the @dynamic to @synthesize so I can use KVC on my properties/ivars?

Perhaps I'm completely wrong in trying to use one of my Core Data entities in some parts of my code without using the core data APIs. Still rather new to it to understand when I can go my own route.

Answer 1

You don't need instance variables with Core Data. A managed object stores these values elsewhere.

I've never gotten very far with Core Data, but if I remember correctly, you're supposed to use primitiveValueForKey: and setPrimitiveValue:forKey: to perform these accesses from your accessor methods. (Outside of your accessors, you should be using either the accessors themselves or valueForKey:/setValue:forKey: instead.)

Answer 2

The reason it doesn't use ivars is that that data isn't there. When you load an object it may not fault in all of its data. ivars are just variables, if you and write to them NSManagedObject doesn't have a chance to fault in the value if it has not been loaded yet. Via accessors NSManagedObject has a choke point that allows it to read the data off the disk if it has not been faulted in yet, which means your object graph can be brought in lazily. Otherwise you would need to bring in every connected object.

Because of that, you can't just add ivars, they won't have the values you want. Likewise you can't change from @dynamic to @synthesized, things will not behave correctly. The dynamic property implementations provided by CoreData are completely KVC and KVO compliant anyway.

If you want to access the values without tripping KVO or KVC you can access the "primitive" values. You do that in one of two ways. You use primitiveValueForKey: and setPrimitive:valueForKey: or you can just declare the primitive and let CD provide dynamic implementations (from the documentation)

@interface Department : NSManagedObject
{
}
@property(nonatomic, retain) NSString *name;
@end

@interface Department (PrimitiveAccessors)
- (NSString *)primitiveName;
- (void)setPrimitiveName:(NSString *)newName;
@end