Question

I have entity Question. Entity Question has property Number. I need to fetch every 20th question: 1, 21, 41, 61, etc. How can I do this?

Answer 1

Create an NSFetchRequest as usual, as well as an NSPredicate instance that uses a modulus operator, e.g. the following untested predicate:

NSPredicate *idPredicate = [NSPredicate predicateWithFormat:@"modulus:by:(Number, 20) == 1"];

Add the predicate to the fetch request, and do the fetch, as usual.

Or create an NSSet of ID values that include {1, 21, 41, ...} and use that with the predicate:

NSSet *idSet = /* set of NSNumber instances including 1, 21, etc. */
NSPredicate *idPredicate = [NSPredicate predicateWithFormat:@"Number IN %@", idSet];

I haven’t tested this, but either approach should get you close, I think.