ios - Why is CoreData ignoring relationshipKeyPathsForPrefetching? -

any ideas why isn't working, items faults.

nsmanagedobjectcontext *tmpcontext = [[nsmanagedobjectcontext alloc] initwithconcurrencytype:nsprivatequeueconcurrencytype]; tmpcontext.parentcontext = self.managedobjectcontext;  nsfetchrequest *request = [nsfetchrequest fetchrequestwithentityname:@"bag"]; request.returnsobjectsasfaults = no; request.relationshipkeypathsforprefetching = @[@"items"];  nsarray *bags = [tmpcontext executefetchrequest:request error:nil];  bag *bag = (bag *)[bags firstobject]; nslog(@"%@", bag.items); 

and get...

relationship 'items' fault on managed object (0x79e90e00) 

and if view coredata query there no join in it.

select t0.z_ent, t0.z_pk, t0.z_opt, .... zbag t0 t0.z_ent = ?