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To ensure that iOS Core Data fetches an entity that has just undergone a relationship update, you can either perform a fetch request with sorting and filtering options, or you can use an NSFetchedResultsController.

1. Fetch Request with Sorting and Filtering: You can perform a fetch request with sorting and filtering options to ensure that the fetched entity fulfills your criteria. For example, if you have a one-to-many relationship between entity A and entity B, and you want to fetch all instances of entity A that have a specific instance of entity B associated with them, you can use the following code:

let fetchRequest: NSFetchRequest<EntityA> = EntityA.fetchRequest()
fetchRequest.predicate = NSPredicate(format: "relationshipToB == %@", specificBInstance)
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "attributeName", ascending: true)]

1. NSFetchedResultsController: You can also use an NSFetchedResultsController to ensure that you are always working with the latest version of your data. With an NSFetchedResultsController, you can specify a delegate which will be notified whenever the underlying data changes, and the fetched results will be automatically updated to reflect those changes. For example, you can use the following code to create an NSFetchedResultsController:

let fetchRequest: NSFetchRequest<EntityA> = EntityA.fetchRequest()
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "attributeName", ascending: true)]
let controller = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
controller.delegate = self
do {
    try controller.performFetch()
} catch let error {
    print("Could not fetch: \(error.localizedDescription)")
}

By using either of these methods, you can ensure that you are always fetching the latest version of your data, including any changes to relationships.