Under the Hood of Core Data - Core Data - page 21

NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@ "name"
ascending:YES];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];

In this example, we are retrieving all the Recipe entities by creating an
NSFetchRequest with the NSEntityDescription set to our entity and no predicate.
However, in addition to fetching the Recipe entities, we also want them sorted.
We accomplish the sorting by adding an NSArray of NSSortDescriptor instances
directly to the NSFetchRequest , which cause the returned NSArray to be properly
sorted.

The NSSortDescriptor takes two parameters as part of its -init : a key and a BOOL
denoting whether the sort is ascending or descending. We can have as many
NSSortDescriptor objects as we want as part of the sort, and therefore they are
placed within an NSArray prior to the sort being performed.

Adding an NSSortDescriptor is especially useful on Cocoa Touch because the
NSFetchedResultsController continues to keep its results sorted without any inter-
vention on our part. The NSFetchedResultsController is discussed in more depth in
Chapter 2, iOS: NSFetchedResultsController , on page 23 .

1.7 Fetched Properties

In addition to NSFetchRequest objects, we have the ability to define a lazy rela-
tionship between objects. Fetched properties are kind of a cross between
relationships and the NSFetchRequest . A fetched property is not a relationship
in the strictest sense and is not realized until the property is requested. When
the property is accessed, Core Data performs the underlying NSFetchRequest
and returns the result. Unlike a normal relationship, a fetched property is
returned as an NSArray as opposed to an NSSet .

In practice, I have found fetched properties to be less useful and less flexible
than either creating a stored fetch request or building the fetch request in
code. Usually when a situation calls for a fetched property, it tends to be
easier to subclass the entity in question, perform an NSFetchRequest in code,
and return the results.

1.8 Wrapping Up

We covered a large number of the important pieces of Core Data in this
chapter. As we continue to explore Core Data in depth, please use this
chapter as a reference point for the aspects of Core Data and how they all fit
together. By the end of the topic, each of these elements should be very
familiar to you.