Orphaned objects in iOS CoreData

Question

Say I have a CoreData entity type called Player and it has a to-one relationship (purpose) with an entity type called PlayerPurpose. For completeness, say we have an inverse relationship in PlayerPurpose called parentPlayer. Consider the following swift code:

// Assume we already have a player object in a NSManagedObjectContext called context:
player.purpose = NSEntityDescription.insertNewObjectForEntityForName("PlayerPurpose",
                 inManagedObjectContext: context) as PlayerPurpose;

// Later in the code, we set the value to nil (or we could have replaced
// it with another call to insertNewObjectForEntityForName)
player.purpose = nil;
// What happens to the previous playerPurpose object within the Managed Object Context?

My question: what happens to the original playerPurpose object within the Managed Object Context when the only reference it has in the data is set to nil (or replaced with another object)?

This is not really related to relationship deletion rules because I'm not explicitly deleting any object -- I'm removing it from any meaningful relationships, making it an orphan.

From an ARC perspective (if the PlayerPurpose was just a normal, non-managed object), the original PlayerPurpose instance now has no references, so it can be cleared from memory -- but what happens in the Managed Object Context? Does CoreData recognize this as an orphaned object and delete it via the context?

If not, then I assume I have to be careful to delete any managed object created via a context if I'm going to get rid of all references to it. Assuming that's the case, is there a good pattern go use for making sure that orphaned objects get cleared from the NSManagedObjectContext and that they are no longer stored in the persistent store?

Thanks!

Tom Harrington Tom Harrington · Accepted Answer · 2014-11-17T18:21:10

Core Data does not automatically delete objects in this scenario, because "orphaned" is a concept that your code has but not one that Core Data recognizes. There's no reason for it to delete a PlayerPurpose object just because one of its relationships is nil.

The most reliable way to ensure that PlayerPurpose instances are deleted would be to

Create custom NSManagedObject subclasses for your entities (if you don't have them already).
Override the setter method for purpose on the Player subclass. If the new value is nil, delete the old one.

You can also handle this by just making sure to call deleteObject: at the appropriate times. Or you could run a clean-up step where you fetch every PlayerPurpose with a nil value for parentPlayer and delete them.

Orphaned objects in iOS CoreData

1 Answers