Handling duplicate entries in Core Data

Question

I have an app that allows users to save favorites. I am using Core Data to store the favorites as managed objects. I have written some code to prevent the possibility of storing

Answer 1

If you're dealing with multiple records, iterating over a count fetch or retrieving actual objects is VERY costly on the CPU. In my case, I did one fetch for all matching records, but asked for a dictionary of just the string of the UUID back. Saves a lot of CPU overhead.

For example, I have a uUID property on every record in core data. I have a corresponding UUID listed as @"UUID" in CloudKit.

  //1. Create a request for the entity type, returning an array of dictionaries  
      NSFetchRequest* request = [NSFetchRequest fetchRequestWithEntityName:@"someEntityName"];
      [request setResultType:NSDictionaryResultType];
      [request setReturnsDistinctResults:YES];
      [request setPropertiesToFetch: @[@"uUID"]];

  //2. Create an array of UUID strings of the downloaded objects
      NSMutableArray *UUIDstrings = [NSMutableArray new];
      for (CKRecord *record in ckRecords) {
        [UUIDstrings addObject:record[@"UUID"]];
      }

   //3. Create a predicate to find any Core Data objects with the same UUID
      [request setPredicate:[NSPredicate predicateWithFormat:@"uUID in %@", UUIDstrings]];

   //4. If there are results from the fetch, do a log and you'll see it's a dictionary. 
      NSArray *deck = [self.MOC executeFetchRequest:request error:nil];

      NSLog(@"Logging the result of index 0. Should be a dictionary %@", deck.count > 0 ? [deck objectAtIndex:0] : @"No results");

   //5. Then either do an embedded fast enumeration (for xx in xx){for xx in xx} to find a match like         

           if ([(NSString *)record[@"UUID"] isEqualToString:[dict valueForKey:@"uUID"]]) 
          {do something}

   //...Or 6. Use a more linear approach with NSSet

    //Harvest the core data strings
      NSMutableArray *coreDataStrings = [NSMutableArray new];
        for (NSDictionary *dict in deck) {
        [coreDataStrings addObject:[dict objectForKey:@"uUID"]];
      }

   //Create a set of your downloaded objects
      NSSet *arraySet = [NSSet setWithArray:ckRecords];

   //Then use a predicate search - a NOT version of above
     NSArray *final = [[arraySet filteredSetUsingPredicate:[NSPredicate predicateWithFormat:@"NOT(UUID in %@)", coreDataStrings]]allObjects];

The console log of the dictionary will look something like this. Just the smallest amount of info required to match:

dictionary {
   uUID = "AFACB8CE-B29E-4A03-9284-4BD5F5464";
}

More here at the developer site on finding unique values.