Adding custom methods to a subclassed NSManagedObject

Question

I have a Core Data model where I have an entity A, which is an abstract. Entities B, C, and D inherit from entity A. There are several properties defined in entity A which are used by B, C, and D.

I would like to leverage this inheritance in my model code. In addition to properties, I am wondering if I can add methods to entity A, which are implemented in it's sub-entities.

For example:

1. I add a method to the interface for entity A which returns a value and takes one argument
2. I add implementations of this method to A, B, C, D
3. Then, I call executeFetchRequest: to retrieve all instances of B
4. I call the method on the objects retrieved, which should call the implementation of the method contained in B's implementation

I have tried this, but when calling the method, I receive:

[NSManagedObject methodName:]: unrecognized selector sent to instance

I presume this is because the objects returned by executeFetchRequest: are proxy objects of some sort.

Is there any way to leverage inheritance using subclassed NSManagedObjects?

I would really like to be able to do this, otherwise my model code would be responsible for determining what type of NSManagedObject it's dealing with and perform special logic according to the type, which is undesirable.

Any help is appreciated, thanks in advance.

Answer 1

If you're getting that exception, it means Core Data is not using your custom class. The key here is NSManagedObject -- that's the object Core Data created for the objects in your data store.

If you haven't already, you'll need to create a class that inherits from NSManagedObject, add your custom methods there, and then set entity A to use your custom class in the object model tool. If entities B, C, D, etc. have specific behaviors, you should subclass the class you created for entity A and assign those entities to use the subclasses too.

Essentially, you have a parallel hierarchy: one hierarchy of entities, and another of classes. You'll likely end up with entity X and class X for each entity in your object model.

Answer 2

It should work. The objects returned by executeFetchRequest: are real instances of NSManagedObjects (or subclasses thereof.) The steps to use custom classes in CoreData are as follows. Say you have entities A and B, where B inherits from A. Then you need two custom classes as

@interface A:NSManagedObject{
}
-(void)someMethod:(NSString*)a;
@end;
@interface B:A{
}
-(void)someMethod:(NSString*)a;
@end;

Then set them in the XCode data modeler as shown:

This way, the CoreData automatically assigns the correct class to the NSManagedObject when it is fetched from the database.

Answer 3

Thanks Alex, that was exactly the missing link that was driving me insane! Thought I was losing it for a while there...