CoreData relationship between objects - ios

Question

I have two kind of objects that I retrieve from a server:
1) Companies
2) Categories
I am updating the data from the server each time that the app is become active(deleting the old data and inserting the new data). From my database Categories and Companies are connected between them with the category_id.
My question here is can I connect with a relationship each company with a category using the category_id?

Answer 1

I assume that means that each Company has a relationship with only one particular Category.
You want to add categoryId to your Category entity, and save the ID when you add the category.
Your models would include a number of other attributes, but here is how the relationships would be setup.
And, the relationships in the inspector...
Of course, you should specify any other options as you deem necessary.
Now, you set the category relationship of a Company entity by searching for the Category based on CategoryId, and then simply setting the category relationship. Core data will automatically take care of the inverse relationship.
Now, given any Category, you can know all the Company objects that are in that category with the `categories relationship, and given any Company you can know which Category it is in.
To fetch a Category by ID, you could do something like this...
- (NSManagedObject*)categoryWithCategoryId:(NSString*)categoryId
inMOC:(NSManagedObjectContext*)moc
error:(NSError **)error {
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:#"Category"];
fetchRequest.predicate = [NSPredicate predicateWithFormat:#"categoryId = %#", categoryId];
fetchRequest.fetchLimit = 1;
return [[moc executeFetchRequest:fetchRequest error:error] firstObject];
}
And to set the relationship, if you have xcode/mogenerator generated subclasses, just set the relationship property.
NSError *error = nil;
Category *category = [self categoryWithCategoryId:categoryId inMOC:moc error:&error];
if (category) {
// If you have generated subclasses...
company.category = category;
// Otherwise, you can do...
[company setValue:category forKey:#"category"];
}