How to properly set relationships in Core Data when using setValue and data already exists

Question

Let's say I have two objects: Articles and Categories with a many-many relationship between the two. For the sake of this example all relevant categories have already been added to the data store. When looping through data that holds edits for articles, there is category relationship information that needs to be saved.

I was planning on using the -setValue method in the Article class in order to set the relationships like so:

- (void)setValue:(id)value forUndefinedKey:(NSString *)key {
    if([key isEqualToString:@"categories"]){
        NSLog(@"trying to set categories...");
    }
}

The problem is that value isn't a Category, it is just a string (or array of strings) holding the title of a category. I could certainly do a lookup within this method for each category and assign it, but that seems inefficient when processing a whole bunch of articles at once. Another option is to populate an array of all possible categories and just filter, but my question is where to store that array? Should it be a class method on Article? Is there a way to pass in additional data to the -setValue method? Is there another, better option for setting the relationship I'm not thinking of?

Thanks for your help.

Answer 1

It looks like you have a 1:1 relationship defined, so you are trying to say that the article belongs to one and only one category.

When you populate one object you may need to create the other; so when an article is created you will need to check your core data store for an object of the correct type and if it does not exist you create it. When you have the object, either pre-existing or just created, you either assign (relationship of one) it to the relationship ivar Category* or (relationship of many) add it to the NSSet* expressing the relationship.

So yes you do need to lookup and assign an actual object, since you must create a relationship to an object in the managed object context, and if it doesn't exist it must be created before that relationship can be made.

Answer 2

Why are you implementing setValue:forKey:? You should have a relationship between these two objects that is set up as many to many and work with the relationship directly. It appears here that that you are doing this the hard way.

What is your end goal with this code?

Update

If I understand you correctly, you have a blob of JSON data and you want to turn that into an object graph and it matches that object graph correct? If so, please look at the code I wrote in this other answer:

http://stackoverflow.com/questions/2362323/json-and-core-data-on-the-iphone/2363996#2363996

In that answer I show how to recursively construct and deconstruct a NSManagedObject graph to and from a JSON structure. It is quite a bit easier than the path you are going down right now.