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copied!<p>I see two ways of doing this. It seems to me that the latter (your proposed method) is the better solution.</p> <p>I changed id to primaryKey since I don't think it would be a good idea to use id as a variable or method name in Object-C since it's a keyword. I might work, I've never really tried. Also I assumed primaryKey to be an NSNumber, since that is how would be stored in Core Data.</p> <p>Method One would execute a fetch request on the context each time:</p> <pre><code>for (id data in someSetOfDataToImport) { NSFetchRequest * request = [[NSFetchRequest alloc] init]; [request setEntity:[NSEntityDescription entityForName:@"Story" inManagedObjectContext:context]]; [request setPredicate:[NSPredicate predicateWithFormat:@"primaryKey = %d", primaryKey]]; NSUInteger count = [context countForFetchRequest:request error:nil]; [request release]; if (count > 0) continue; // Insert data in Managed Object Context } </code></pre> <p>Method Two does what you proposed, cache the keys in an array and check it instead of going to the source:</p> <pre><code>NSFetchRequest * request = [[NSFetchRequest alloc] init]; [request setEntity:[NSEntityDescription entityForName:@"Story" inManagedObjectContext:context]]; NSArray * allStories = [context countForFetchRequest:request error:nil]; [request release]; NSMutableArray * allPrimaryKeys = [[allStories valueForKeyPath:@"@distinctUnionOfObjects.primaryKey"] mutableCopy]; for (id data in someSetOfDataToImport) { if ([allPrimaryKeys containsObject:data.primaryKey]) continue; [allPrimaryKeys addObject:data.primaryKey]; // Insert data in Managed Object Context } [allPrimaryKeys release]; </code></pre>

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