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copied!<h2>To recap/Full guide:</h2> <ol> <li><p>Before making any change, create a new model version. </p> <p>In Xcode 4: Select your <code>.xcdatamodel</code> -> Editor -> Add Model Version. </p> <p>In Xcode 3: Design -> Data Model -> Add Model Version. </p> <p><em>You will see that a new <code>.xcdatamodel</code> is created in your <strong><code>.xcdatamodeld</code></strong> folder (which is also created if you have none)</em>.</p></li> <li><p>Save.</p></li> <li><p>Select your new <code>.xcdatamodel</code> and make the change you wish to employ in accordance with <a href="http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreDataVersioning/Articles/vmLightweightMigration.html#//apple_ref/doc/uid/TP40004399-CH4-SW1" rel="noreferrer">the <em>Lightweight Migration</em> documentation</a>.</p></li> <li><p>Save.</p></li> <li><p>Set the current/active schema to the newly created schema. </p> <p>With the <strong><code>.xcdatamodeld</code></strong> folder selected:</p> <p>In Xcode 4: Utilities sidebar -> File Inspector -> Versioned Core Data Model -> Select the new schema. </p> <p>In Xcode 3: Design > Data Model > Set Current Version. </p> <p><em>The green tick on the <code>.xcdatamodel</code> icon will move to the new schema.</em></p></li> <li><p>Save.</p></li> <li><p>Implement the necessary code to perform migration at runtime. </p> <p>Where your <code>NSPersistentStoreCoordinator</code> is created (usually AppDelegate class), for the <code>options</code> parameter, replace <code>nil</code> with the following code:</p> <pre><code>[NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption, [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption, nil] </code></pre></li> <li><p>Run your app. If there's no crash, you've probably successfully migrated :)</p></li> <li><p>When you have successfully migrated, the migration code (step 7) can be removed. (It is up to the developer to determine when the users of a published app can be deemed to have migrated.)</p></li> </ol> <p><strong>IMPORTANT:</strong> Do not delete old model versions/schemas. Core Data needs the old version to migrate to the new version.</p>

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