Question

I have a managed object reference *event.

An Event has many Occurrences. An Occurrence has an attribute call date

I want to fetch all the occurrences of the event *event which has today’s date. How should I write a NSPredicate on a FetchRequest.

Answer 1

You have to be aware, that there are two ways to get your Occurrence objects: You can traverse the relationship from the Event object (i.e. myEvent.occurences) or you can create a fetch request for all occurrences that have the event of interest in their inverse relationship, i.e. a predicate like [NSPredicate predicateWithFormat:@"event == %@", myEvent"].

There are fundamental differences to these two. A fetch request will always cause SQL to be executed and I/O to happen which is expensive. If you simply traverse the relationship, and you’re in a situation where the Occurrence objects are already in memory, it will be very cheap to do (no SQL, no I/O). The fetch request on the other hand, will allow you to fault in all objects in one go, while traversing will only fault objects one at a time, so you’ll end up doing multiple SQL + I/O events. Also, a fetch request allows you to limit the predicate even further. But in many cases, simply traversing might be the best case.

CoreData does a lot of heavy lifting for you, but at the end of the day, you still need to know about when to do SQL and I/O and how to prevent it.

I hope this helps.