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Data: advanced techniques
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initWithKey:@"creationDate" ascending:NO]; NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil]; [request setSortDescriptors:sortDescriptors]; [sortDescriptors release]; [sortDescriptor release];
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NSError *error; NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy]; if (mutableFetchResults == nil) { NSLog(@"Error fetching result %@",[error description]); } [self setEntries:mutableFetchResults]; [mutableFetchResults release]; [request release];
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The first step in retrieving results from the database is to create the fetch request. After the request has been created, you must set its entity. The entity represents which object type you re retrieving. In this case, the entity is an Entry of your journal. After your request is created, you must tell it how to sort the results. If you omit this step, the ordering of the results will be undefined. This means the results returned could be in any order. The sort descriptor you create in B tells the request to sort the results by the creation date in ascending order. You can sort based on any field in your entity. The last thing you need to do is execute the request C. Notice that the request returns an NSMutableArray. This array contains all the objects retrieved from the database in the order specified by the sort descriptor. To keep these results around, you set them to a class variable D. When you have an array of objects on hand, you can begin modifying or deleting them. Let s look at modifying objects. Here s how you update a managed object:
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- (void) update:(Entry *) entry { [entry setTitle:textField.text]; [entry setBody:textView.text]; [entry setCreationDate:[NSDate date]]; NSError *error; if (![self.managedObjectContext save:&error]) { NSLog(@"Error Saving: %@",[error description]); } }
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As you can see, the code is almost identical to the code to add a new entry. The only difference is how the entry is retrieved. Instead of letting Core Data allocate a new entry for us, you modify one you already have on hand. Typically, this is first retrieved from the array you created in listing 9.13. As with updating, you must have a managed object on hand in order to delete it. You can t delete a managed object without first retrieving it. Here s how you delete a managed object from the database:
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Summary
-(void) delete:(Entry *) entry { [managedObjectContext deleteObject:entry]; [entries removeObject:entry]; NSError *error; if (![managedObjectContext save:&error]) { NSLog(@"Error deleted entry %@",[error description]); } }
The delete method is fairly straightforward. The first thing to do is remove the object from the managed object context. Any time the context is saved after removing the object, it deletes that object from the data store. Next, you delete the object from the global array of entries so you can reflect the update to the user. If you don t do this, the user may still see the object in a table view, even though it s deleted forever when the application exits. Finally, you save the context. As with any changes made to the context, saving it makes them permanent.
Summary
As you ve seen, you have two powerful options to consider when storing large amounts of data on the iPhone and iPad. SQLite is great for anyone with prior experience with SQL and MySQL. You have the ability to use full SQL syntax to work with the records without having to learn a new design pattern. Core Data is Apple s response to solving the complexities associated with SQL. You no longer need to know complicated SQL syntax in order to have a fully functional database in your application. Core Data extracts much of the process and gives you high-level objects to work with as you please. In the next chapter, we ll move away from data storage and work with some of the cool hardware features of the iPhone and iPad. These include the accelerometer, GPS, and compass.
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