Objective-C Fundamentals

Beyond the basics
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All you do is pass in two options that tell Core Data to try to automatically determine
what has changed between the data models and to try to automatically migrate the data.
Build and run it again—everything should work.
If you can, you should try to avoid substantial changes to your data model, especially
after you’ve shipped your application and people are already using it. Put some
time and thought into your data model before you start coding.
If you need to perform a more complex migration, check out Apple’s Core Data
Model Versioning and Data Migration Programming Guide (http://developer.apple
.com/iphone/library/documentation/Cocoa/Conceptual/CoreDataVersioning/
Introduction/Introduction.html).
12.5.2 Performance
Using NSFetchedResultsController is already one of the best things you can do
regarding performance of your iOS Core Data application. There are a few more
things to keep in mind, though. If you know that you’ll always need access to a certain
relationship of your entity, you should tell Core Data to fetch those child entities with
the initial fetch request. Otherwise, Core Data would have to execute another fetch
request for every entity. With 100 people, that would make 101 fetch requests (the initial
one to fetch the people and one for each person to get the tasks). Using
NSFetchResult’s setRelationshipKeyPathsForPrefetching: method avoids this overhead.
In your example, to improve the performance when you fetch people, it would
look like this:
[request setRelationshipKeyPathsForPrefetching:[NSArray
arrayWithObject:@"tasks"]];
This code loads the tasks right away along with the people.
Another performance concern is binary data: unless the data is very small (smaller
than 100 kb), you shouldn’t store it directly in the entity it belongs to. If the data is
bigger than 100 kb, consider storing it in a separate entity and setting up a relationship
between the data entity and the entity it belongs to (maybe a profile picture that
belongs to a person). Anything bigger than 1 MB should not be stored in a Core Data
entity but on disk as a file. Only the path to the file should be stored using Core Data.
More information about performance can be found in Apple’s Core Data Programming
Guide: http://developer.apple.com/mac/library/documentation/Cocoa/
Conceptual/CoreData/Articles/cdPerformance.html.
12.5.3 Error handling and validation
You undoubtedly noticed that every time you save the managed object context, you
pass in a pointer to an NSError object. So far, you haven’t really handled those errors.
In a real application users have a right to be informed about anything that goes wrong
with their data. So how do you handle errors in Core Data?
If you encounter an error from which you can’t recover (can’t write data to disk or
something catastrophic like that), you should inform the user about it with an