Changes not recognized my NSFetchedResultsController attached to background thread

Question

I am using MagicalRecord, but not sure if this is a bug in MagicalRecord, or CoreData.

I create new entities in my UI thread when a user clicks the new button and it gets saved to the default context:

- (IBAction) saveToCoreData:(UIButton *)sender {
    NSManagedObjectContext *context = [NSManagedObjectContext MR_defaultContext];
    Report *report = [Report MR_createInContext:context];
    report.dirty = [NSNumber numberWithBool:YES];
    report.timestamp = [NSDate date];

    Document *document = [Document MR_createInContext:context];
    document.dirty = [NSNumber numberWithBool:YES];
    document.timestamp = [NSDate date];
    document.report = report;

    NSLog(@"Saving.................................");
    [context MR_saveToPersistentStoreWithCompletion:^(BOOL success, NSError *error) {
        NSLog(@"Saved report and document to PS");
    }];
}

Notice I have a Report entity with a child Document attached

Then I have a background thread that is only responsible to listen for changes, if a Report changes, dirty == YES then it needs to get submitted to the server.

I set up a NSFetchedResultsController on a background thread to listen for changes so I know when save to server.

// This should get me a new background context, use it in FRC below
    self.fetchedResultsController = [Report MR_fetchAllSortedBy:@"timestamp"
                                                      ascending:NO
                                                  withPredicate:[NSPredicate predicateWithFormat:@"dirty == YES"]
                                                        groupBy:nil
                                                       delegate:self
                                                      inContext:_managedObjectContext];

Then I implement the method to listen for changes:

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id) anObject atIndexPath:(NSIndexPath *) indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *) newIndexPath {
    switch(type) {

        case NSFetchedResultsChangeInsert:
            NSLog(@"report inserted");
            [self pushReport:anObject];
            break;
        case NSFetchedResultsChangeUpdate:
            NSLog(@"report updated");
            [self pushReport:anObject];
            break;
    }
}

and pushing it to the server:

- (void) pushReport:(Report *) report {
    __weak ReportPushService *weakSelf = self;
    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
    [manager GET:@"http://echo.jsontest.com/" parameters:nil success:^(AFHTTPRequestOperation *operation, id responseObject) {
        [weakSelf.managedObjectContext performBlockAndWait:^{
            report.remoteId = @"12345"; // fake id from server
            report.dirty = [NSNumber numberWithBool:NO];

            [weakSelf.managedObjectContext MR_saveToPersistentStoreWithCompletion:^(BOOL success, NSError *error) {
                NSLog(@"Saved server info to Report");
            }];
        }];
     } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
         NSLog(@"Error: %@", error);
     }];
}

The FRC works like a charm and when the UI thread adds a new report with dirty == YES the FRC picks it up and submits it to the server.

Here is where I run into problems. After the report is saved to the server, the server return an ID for that report. I set that as report.remoteId.

I have a FRC in another class that is responsible for saving documents to the server. It will check the documents dirty == YES and that it parent (Report) remoteId != nil, ie:

    self.fetchedResultsController = [Document MR_fetchAllSortedBy:@"timestamp"
                                                      ascending:NO
                                                  withPredicate:[NSPredicate predicateWithFormat:@"report.remoteId != nil && dirty == YES"]
                                                        groupBy:nil
                                                       delegate:self
                                                      inContext:_managedObjectContext];

It uses the same background MOC as the FRC controller in the Report "push" class.

I then listen for changes on that FRC:

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id) anObject atIndexPath:(NSIndexPath *) indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *) newIndexPath {
    switch(type) {
        case NSFetchedResultsChangeInsert:
            NSLog(@"document inserted");
            [self pushDocument:anObject];
            break;
        case NSFetchedResultsChangeUpdate:
            NSLog(@"document updated");
            [self pushDocument:anObject];
            break;
    }
}

However his FRC never sees any changes, even though I have verified that I have a report with remoteId != nil and a (child) document with dirty == YES.

Why is the Documents FRC not seeing objects that match that predicate?

Answer 1

The problem / limitation with FRC is that it only contains the entities for which it was setup. In this case:

self.fetchedResultsController = [Document MR_fetchAllSortedBy:@"timestamp"
                                                      ascending:NO
                                                  withPredicate:[NSPredicate predicateWithFormat:@"report.remoteId != nil && dirty == YES"]
                                                        groupBy:nil
                                                       delegate:self
                                                      inContext:_managedObjectContext];

the FRC will only contain Document entities. The initial fetch will work on the report relation. But if a report changes the FRC will not be updated, even though the predicate matches.

http://www.mlsite.net/blog/?p=825

and here

Changing a managed object property doesn't trigger NSFetchedResultsController to update the table view