Navigating the Core Data Object Graph [2]

Question

I asked a question yesterday where I really should have started with a simpler example. Having distilled my question to the basics, I\'ve managed to solve my problem using exis

Answer 1

I've never tried to use a subquery before, that's very cool and good to see an example of it. You should add the solution as an answer to the question actually!

I would have probably done as Abizern suggested, since this is a tree-like hierarchy and its easier to traverse up the tree using to-one relationships.

In code this looks like:

NSManagedObjectContext *moc = APPDELEGATE.managedObjectContext;
NSFetchRequest *request = [NSFetchRequest new];
[request setEntity:[NSEntityDescription entityForName:@"EntityC" inManagedObjectContext:moc]];
[request setPredicate:[NSPredicate predicateWithFormat:@"tag = YES"]];
NSError *fetchError = nil;
NSArray *children = [moc executeFetchRequest:request error:&fetchError];

children is an array of EntityC objects that match the predicate. The next step is getting a set of unique EntityA objects that are the "grandparents" of these. We can take advantage of key-value coding here:

NSArray *grandParents = [children valueForKeyPath:@"parent.@distinctUnionOfObjects.parent"];

In this case, for efficiency, we'd probably want to prefetch the parent.parent keypath during our initial fetch request:

[request setRelationshipKeyPathsForPrefetching:@[@"parent.parent"]];

Hope this helps!