Question

So I’m using Core Data in an existing iPhone app, and I’ve set up two entities: Person and Item. The root view of my app’s navigation (currently) shows a list of people, and you drill down from there to items. In short, the hierarchy looks like this:

Person -> Item

I want to add a new entity above Person in the hierarchy, called List:

List -> Person -> Item

Additionally, I want the user’s first List to be created for them on startup, and for any People the user’s already added to be assigned to that list.

I’m familiar with Core Data’s lightweight migration & versioning feature, so I think I know how to add the new entity and relationship, but I’m not sure how to:

1. Create a List record on app start if they’ve never had the Lists feature before

2. Set all existing People records to belong to that new list.

One quick and dirty way would be to add some code to my app delegate’s applicationDidFinishLaunching method that performs the migration by (1) checking to see if there are any Lists, (2) if not, creating the default one, (3) fetching all existing People from my data store, (4) setting each Person’s list attribute to the newly created default list, and finally (5) saving those changes.

My question is: is there any faster or easier way to do all of that?

Answer 1

That’s pretty much what you’d want to do. Use an NSFetchRequest to see if any Listss exist. If not, create one. Then do another request to get all the Persons. Here, instead of assigning the list property of each Person, I’d create an NSSet containing all your Persons and assign that to the List‘s people property. You did create an inverse property, right?

This is actually a pretty lightweight operation, all tolled, so I wouldn’t worry too much about performance. Unless you’ve got hundreds or thousands of Person objects, your user will probably won’t even notice.