Core Data (iPhone)- Limiting the fetch results across a relationship

Question

I have recently been looking at Core Data for the iPhone and I have one query which I have not been able to find an answer to in the literature.

Lets imagine that I have a User model object with a to-many relationship with a Purchase object. A User may have thousands of purchases.

All of the information I have seen thus far seems to suggest that a fetch operation carried out across that relationship would return an NSSet with all of the Purchase objects associated with a particular User.

Clearly I would prefer if there was some way to limit the number of Purchase objects fetched by:

1) imposing some sort of criteria (e.g. only purchases over £1000 etc.); or

2) only fetching in batches if the above is not possible.

Is the above possible? I am just concerned that with the limited memory of the iPhone that I risk overloading it with thousands of unnecessarily fetched objects.

Thanks in advance for any replies.

Answer 1

My understanding is that when you fetch User objects, the relationship to its Purchase objects will be a fault that will only be loaded if accessed.
Instead, you could fetch Purchase objects with a predicate that specifies the User(s) and check(s) on any other criteria.

That said, I would definitely test realistic use cases (on devices) before spending too much time on performance issues. The efficiency of the iPhone and Core Data may surprise you.

Answer 2

1. Use a fetched property
2. Use NSFetchedResultsController

The answer to these and the questions you will have next are answered in the Core Data Programming Guide for the iPhone.