Question

The problem I’m having is that i can’t assign to an attribute of a subclass unless i’ve set the subclassed entity’s Parent Entity property to the entity of the abstract superclass.

In XCode 4.0.2, this is the Parent Entity property to which i’m referring:

What i don’t get is that i thought a Parent Entity is intended for parent-child relationships between entities and the object relationships are simply captured by the class definitions.

Example

Entities A, B and C:

• A is an abstract entity of type A with attributes:
  • y : string
  • z : string
• B is an entity of type A with no attributes
• C is an entity of type A with no attributes

Classes A, B and C:

@interface A : NSManagedObject {
}
@property (nonatomic, retain) NSString * y;
@property (nonatomic, retain) NSString * z;

@interface B : A {
}
@interface C : A {
}

The Problem

If i don’t set the Parent Entity for entities B and C to be entity A, then when i try:

NSEntityDescription *be = [[mom entitiesByName] objectForKey:@"B"];
B *b = [[NSManagedObject alloc] initWithEntity:be
                insertIntoManagedObjectContext:moc];

b.y = @"test"; // <<-- This line causes the following error:

I get:

-[NSManagedObject setY:]: unrecognized selector sent to instance

If i set the Parent Entity, it seems to work except that entity persisted to the store seems to be A instead of B.

Answer 1

Did you remember to set the class for entity B to class B? If yes, you should take a look at the type of the pointer that’s assigned to b… is it in fact a B*? It looks like it’s probably a NSManagedObject* based on the error you’re getting, and according to the docs for -initWithEntity:insertIntoManagedObjectContext:…

The dynamically-generated subclass
will be based on the class specified
by the entity, so specifying a custom
class in your model will supersede the
class passed to alloc.

…I think you should be getting a B* if your model is configured correctly.