Question

Most (all I’ve seen) Core Data tutorials use the following code snippet with @"MyEntityClass" hard-coded in:

NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"MyEntityClass"];

Is it safe to use NSStringFromClass() as an Entity Name?

NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:NSStringFromClass([MyEntityClass class])];

This seams to be much easer to deal with regarding refactoring and the like. Especially since I am having Xcode create my NSManagedObject subclasses. I ask because I have never seen this before, so perhaps I am missing something.

Answer 1

Yes, that code is fine, if your entity’s class is set to MyEntityClass in your model.

I prefer to give the entity class a class method that returns the entity name:

+ (NSString *)entityName {
    return NSStringFromClass(self);
}

and call it like this:

NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:[MyEntityClass entityName]];

This way, if I want to change the class name without changing the entity name in the model, I can just make the change in the class method:

+ (NSString *)entityName {
    return @"NewEntityName";
}

Why would I do that? Well, I might decide on a better name for the entity. Changing the class name doesn’t break compatibility with an existing Core Data persistent store, but changing the entity name in the model file does. I can change the class name, and the entityName method, but leave the entity name unchanged in the model, and then I don’t have to worry about migration. (Lightweight migration supports renamed entities so it’s not that big of a deal either way.)

You could go further and actually have the entityName method look up the entity name from the managed object model at runtime. Suppose your application delegate has a message that returns the managed object model:

+ (NSString *)entityName {
    static NSString *name;
    static dispatch_once_t once;
    dispatch_once(&once, ^{
        NSString *myName = NSStringFromClass(self);
        NSManagedObjectModel *model = [(AppDelegate *)[UIApplication delegate] managedObjectModel];
        for (NSEntityDescription *description in model.entities) {
            if ([description.managedObjectClassName isEqualToString:myName]) {
                name = description.name;
                break;
            }
        }
        [NSException raise:NSInvalidArgumentException
            format:@"no entity found that uses %@ as its class", myName];
    });
    return name;
}

Obviously, if you really want to do this, you should factor out the contents of the dispatch_once block into a helper method, probably on your app delegate (or wherever you get the model).