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POHow does this code use only updates into core data?
 

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  1. POHow does this code use only updates into core data?
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    <p>This is a SyncEngine from an RW tutorial. I need help understanding how only UPDATED records from the web are fetched and processed into Core Data.</p> <pre><code>- (void)processJSONDataRecordsIntoCoreData { NSManagedObjectContext *managedObjectContext = [[SDCoreDataController sharedInstance] backgroundManagedObjectContext]; // Iterate over all registered classes --- CHECK! for (NSString *className in self.registeredClassesToSync) { if (![self initialSyncComplete]) { NSDictionary *JSONDictionary = [self JSONDictionaryForClassWithName:className]; NSArray *records = [JSONDictionary objectForKey:@"results"]; for (NSDictionary *record in records) { [self newManagedObjectWithClassName:className forRecord:record]; } } else { NSArray *downloadedRecords = [self JSONDataRecordsForClass:className sortedByKey:@"objectId"]; if ([downloadedRecords lastObject]) { NSArray *storedRecords = [self managedObjectsForClass:className sortedByKey:@"objectId" usingArrayOfIds:[downloadedRecords valueForKey:@"objectId"] inArrayOfIds:YES]; int currentIndex = 0; //if dl count is &lt; current index, there is an updated object dl from the web for (NSDictionary *record in downloadedRecords) { NSManagedObject *storedManagedObject = nil; //Quick check to see if they indeed match, if they do, update the stored object with remote service objects if ([storedRecords count] &gt; currentIndex) { storedManagedObject = [storedRecords objectAtIndex:currentIndex]; } //Othwerwise its a new object and you need to create a new NSManagedObject to represent it in CDdb if ([[storedManagedObject valueForKey:@"objectId"] isEqualToString:[record valueForKey:@"objectId"]]) { [self updateManagedObject:[storedRecords objectAtIndex:currentIndex] withRecord:record]; } else { [self newManagedObjectWithClassName:className forRecord:record]; } currentIndex++; } } } // After all NSMO are created in your context, save it! [managedObjectContext performBlockAndWait:^{ NSError *error = nil; if (![managedObjectContext save:&amp;error]) { NSLog(@"Unable to save context for class %@", className); } }]; // Cleanup time [self deleteJSONDataRecordsForClassWithName:className]; [self executeSyncCompletedOperations]; } [self downloadDataForRegisteredObjects:NO]; } </code></pre> <p>From what I understand, on the first or initial sync, it fetches JSONDictionaryForClassWithName which reads the downloaded data from disk and creates a newManagedObjectWithClassName.</p> <p>My confusion is in the update else block. downloadedRecords is populated from JSONDataRecordsForClass which simply calls JSONDictionaryForClassWithName. Then it checks to see if there is at least 1 object in that array. If there is it does this:</p> <pre><code>NSArray *storedRecords = [self managedObjectsForClass:className sortedByKey:@"objectId" usingArrayOfIds:[downloadedRecords valueForKey:@"objectId"] inArrayOfIds:YES]; </code></pre> <p>This fetches all managedObjectsForClass:sortedByKey which is below:</p> <pre><code>- (NSArray *)managedObjectsForClass:(NSString *)className sortedByKey:(NSString *)key usingArrayOfIds:(NSArray *)idArray inArrayOfIds:(BOOL)inIds { __block NSArray *results = nil; NSManagedObjectContext *managedObjectContext = [[SDCoreDataController sharedInstance] backgroundManagedObjectContext]; NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:className]; NSPredicate *predicate; if (inIds) { predicate = [NSPredicate predicateWithFormat:@"objectId IN %@", idArray]; } else { predicate = [NSPredicate predicateWithFormat:@"NOT (objectId IN %@)", idArray]; } [fetchRequest setPredicate:predicate]; [fetchRequest setSortDescriptors:[NSArray arrayWithObject: [NSSortDescriptor sortDescriptorWithKey:@"objectId" ascending:YES]]]; [managedObjectContext performBlockAndWait:^{ NSError *error = nil; results = [managedObjectContext executeFetchRequest:fetchRequest error:&amp;error]; }]; return results; } </code></pre> <p>The next bit which compares the [storedRecords count] > currentIndex is confusing. Can someone please explain this? I think my confusion lies in what the managedObjectsForClass method does with the usingArraysOfIds &amp; inArrayOfIds.</p> <p>I would expect that at some point it gets the the updatedAt field from the downloaded records and compares it to the updatedAt field of the CoreData fetched records.</p>
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