Core Data does not update inverse relationship - ios

Question

My two entities are Book and Category. A book can have many categories, and a category can have many books. The relationships are marked as the inverse of each other in the data model.
When I update the categories of a book I expected that the books of the corresponding categories would be updated automatically. In other words:
Book *book = [Book insertNewObjectIntoContext:context];
Category *category = [Category insertNewObjectIntoContext:context];
[book addCategories:[NSSet setWithObject:category]];
BOOL result = [category.books containsObject:book]; // Should be YES
Yet, I get NO in my app. books returns an empty set.
I triple-checked that the relationships are marked as inverse. What else could be at play here? Or is there something I'm missing?
Would it hurt if I set the inverse relation manually? If I do everything works like a charm.

Answer 1

The inverse relationship is not updated until the end of the run loop. Calling it immediately after like that does not give it time to update.
If you were to do something like:
dispatch_async(dispatch_get_main_queue(), ^{
BOOL result = [category.books contains:book];
NSLog(#"Result: %#", result ? #"YES" : #"NO");
});
You would see your expected result.
Update
It does not hurt to set the reverse manually. When everything is working correctly it results in a waste of cycles because Core Data has to double check the relationships so you will see a performance hit.

Answer 2

You can call
[context processPendingChanges];
to force the managed object context to update the inverse relationships immediately.
This method is automatically called during the event loop, or when you save the
context. But you can call it explicitly as well. From the documentation:
Forces the receiver to process changes to the object graph.
...
You can also invoke it manually to coalesce any pending unprocessed
changes.