Can I add properties to subclass of subclass of NSManagedObject?

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I have a class Base : NSManagedObject generated by core data, and in order to add some methods to this entity, I subclass it with Derived : Base. Now, I want to add a property (which is not in Base class) to Derived class. But when I try to access the setter of the added property, it throws an exception:

caught "NSInvalidArgumentException", "-[Base setAddedProperty:]: unrecognized selector sent to instance 0x7fdcc31b36d0"

Could anyone help?

EDIT: It seems that subclass of subclass of NSManagedObject cannot have its own methods because I just added a method to Derived and got the similar exception. Does that mean category is the only way to add methods to subclass of NSManagedObject?

EDIT: I changed the "Class" of entities in configurations to Derived and everything is working now. All those newly added properties can be accessed and customized methods can be invoked. Why? and is this a good practice?

EDIT: It seems the above descriptions are misunderstood. I intend to subclass the entity classes generated by core data in order to provide customized methods and properties, not to create child entities. Therefore, Derived is not an entity in data model.

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@Zhihhao L. Did you try to upgrate your .xcdatamodel? After you add new entity to your .xcdatamodel you need to delete your classes and made it again. – Сергей Олейнич
@СергейОлейнич I didn't change data model. I just generated subclasses of NSManagedObject and created some subclasses to those generated subclasses. – Codinfox
@Zhihhao L. Did you set a checkbox for inheritance in your model? – Сергей Олейнич
@СергейОлейнич No. But my intention is to subclass those generated entity classes to provide some customized properties and methods. Not to create child entities. – Codinfox
@ZhihaoL. you can't store extra 'Derived' properties if you don't add appropriate fields in core data model, but they would be able in runtime if you 'synthesize' getter and setter for them (use '@synthesize' keyword instead of '@dynamic' in implementation file). In this way, core data will save only attributes specified in data model (Base entity), all other will exist only from fetch to fetch – Mykola Denysyuk

3 Answers

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You also should specify inheritance in xcdatamodel . Check fields: Name, Class, Parent Entity in xcdatamodel. Derived : Base isn't enough for NSManagedObjectsubclasses.

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You need to select parent sheep (also try to set abstract entity for you Base) Check this picture and you will see what i mean about checkbox

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I got this problem fixed.

In my .xcdatamodeld file, I mapped Base entity to Base class. In my codes, I create an instance by:

Derived* pointer = [Derived MR_createEntity];

This method invokes NSEntityDescriptor insertNewObjectForEntityForName:inManagedObjectContext for me. However, this method will return an instance of Base instead of Derived despite the pointer is of type Derived*, which should be an syntax error if in compilation.

Therefore, in order to make my methods in Derived valid, I need to manually change the mapping from Base to Derived.

EDIT: After writing this answer, I found an awesome tool mogenerator, which makes use of exactly the same idea as I described above and is really convenient.