iPhone Core Data: Cascading delete across a many-to-one relationship

Question

I have two classes A and B with a many-to-one relationship from A to B (multiple A objects may reference the same B). The question is, if the delete rule on the A side is C

Answer 1

Here's a Swift 4 version of Lukas' answer:

public override func prepareForDeletion() {
    guard let department = department else { return }

    if department.employees.filter({ !$0.isDeleted }).isEmpty {
        managedObjectContext?.delete(department)
    }
}

Answer 2

I had the same goal as you apparently had (delete B as soon as the last referenced A is deleted). It took me longer than expected to get this right. Particularly because

• At the time A prepares for deletion, the to-many relationship in B might not be updated yet, so you can't just count the A referenced in B.
• isDeleted on A seems to be already set during -prepareForDeletion

Here's what worked for me if anybody's interested (I'll use Department <-->> Employee because it's easier to read):

In Employee:

- (void)prepareForDeletion {
    // Delete our department if we we're the last employee associated with it.
    Department *department = self.department;
    if (department && (department.isDeleted == NO)) {
        NSPredicate *predicate = [NSPredicate predicateWithFormat:@"isDeleted == NO"];
        NSSet *employees = [department.employees filteredSetUsingPredicate:predicate];

        if ([employees count] == 0) {           
            [self.managedObjectContext deleteObject:department];
        } 
    }
}

Other people have suggested putting this logic into -willSave in Department. I prefer the solution above since I might actually want to save an empty department in some cases (e.g. during manual store migration or data import).