Question

I am importing data from a web service, in such data I have a tree of categories.

For example:

   Restaurants
     – Italian
     – Mexican
     – Steakhouse

In my model I have set a relation called ParentCategory with an inverse relation called Subcategories. Everything is good so far.

Now, when I am importing my categories I am doing something like this: (Note: My code does not look like this. I have changed it to exemplify my problem)

NSEntityDescription *categoryEntity = [NSEntityDescription entityForName:@"Categories" inManagedObjectContext:context];

NSDescription *parentCategory = [NSEntityDescription insertNewObjectForEntityForName:[categoryEntity name] inManagedObjectContext:context]

//Then I actually set some values for parentCategory.
//Then create the subcategories. 

NSDescription *italianDescription = [NSEntityDescription insertNewObjectForEntityForName:[categoryEntity name] inManagedObjectContext:context];
[italianDescription objectForKey:@"Italian" forKey:@"name"];

//Set the relationship
[italianDescription objectForKey:parentCategory forKey:@"ParentCategory"];

//All this works great but then when 
NSEntityDescription *mexicanDescription = [NSEntityDescription insertNewObjectForEntityForName:[categoryEntity name] inManagedObjectContext:context];
[mexicanDescription objectForKey:@"Mexican" forKey:@"name"];
[mexicanDescription objectForKey:parentCategory forKey:@"ParentCategory"];

It seems like when that last line of code runs it removes the relation from “italianDescription”, therefore after I save my context it shows like it doesn’t have a parent category. I removed some data from my web service and only the last subcategory that I set the relation to parent category object X keeps it. All previous will loose it.

I checked the apple developer documentation and it doesn’t really help. That being said, how can I overcome this? Or what would be a efficient way to import that with relations.

Answer 1

I don’t think you are setting anything, the way to set values for NSManagedObjects is

setValue: forKey:

Instead of NSDescription*’s you should be using NSManagedObject*

You may want to look into subclassing your entities as it allows for much easier access and setting of properties. In which case it would be

MyCustomEntity* myobj=[NSEntityDescription insertNewObjectForEntityForName:@"MyentityName" inManagedObjectContext:context];

Then you can set the values just by using it’s properties with dot notation or messaging. You can do this by selecting the entity in the core-data editor and then under Editor on the Menu bar there is an option to create a NSManagedObject subclass, after you create that you need to tell the entity that it has a custom class now. This is done in the right hand Utiliies bar under Entity there is a Class field, just enter the name of your new class and it should be set up.