FetchedResultsController and displaying data from 3 nested entities in one UITableViewCell - ios

Question

Could somebody explain me how can I display in one tableViewCell data from three entities connected by relationship?
I have 3 entities, let it be User, Device, and Alert. Relationships : User has many Devices and these devices have many Alerts.
In my AlertsTableViewController I want to display every alerts with user_name property which is included in User entity.
Should I start my fetch request by setting entity for name User or Alert?.
- (NSFetchedResultsController *)fetchedResultsController
{
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"????????" inManagedObjectContext:_managedObjectContext];
[fetchRequest setEntity:entity];
// Set the batch size to a suitable number.
[fetchRequest setFetchBatchSize:12];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"created_at" ascending:NO];
NSArray *sortDescriptors = #[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_managedObjectContext sectionNameKeyPath:nil cacheName:#"Alerts"];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error])
{
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
}
return _fetchedResultsController;
}
My TableView methods:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *cellIdentifier = #"AlertsTableViewCell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier forIndexPath:indexPath];
User *u = [[self fetchedResultsController] objectAtIndexPath:indexPath];
//I know how to display User with details of devices by looping nsset but how to display every alerts with User.name?
for (Device *device in u.devices) {
[cell.textLabel setText:device.someProperty];
}
return cell;
}

Answer 1

When you like to display Alerts, in most simple cases (like yours) you will set the fetch request entity to #"Alert".
To select only Alerts that belong to a given user you will need to define a predicate for your fetch request:
fetchRequest.predicate = [NSPredicate predicateWithFormat:#"device.user = %#",<User object or User objectID>];
The User object or objectID will be known to you from the app user selection, or, if you have another unique identifier for each user (like the userName property) you could include that in your predicate:
fetchRequest.predicate = [NSPredicate predicateWithFormat:#"device.user.name = %#",userNameVariable];
In most cases, the LAST thing you want to do is loop over a set of NSManagedObjects as they will fire faults one by one and give you a really bad user experience (especially in cellForRow...).
Get the data you like to display before you need to supply it to the view.
Edit:
In that spirit, if you like to display the someProperty value in your AlertsTableViewController you should also add:
fetchRequest.relationshipKeyPathsForPrefetching = #[#"device"];
So that the Device will be fetched along with Alert object.
Obviously, you have to change your cellForRow... method to accept Alert objects from the FRC.