NSFetchedResultsController with sections created by first letter of a string

Question

Learning Core Data on the iPhone. There seem to be few examples on Core Data populating a table view with sections. The CoreDataBooks example uses sections, but they\'re ge

Answer 1

Here's how you might get it to work:

• Add a new optional string attribute to the entity called "lastNameInitial" (or something to that effect).
• Make this property transient. This means that Core Data won't bother saving it into your data file. This property will only exist in memory, when you need it.
• Generate the class files for this entity.

• Don't worry about a setter for this property. Create this getter (this is half the magic, IMHO)

  - (NSString *) lastNameInitial {
  
       [self willAccessValueForKey:@"lastNameInitial"];
  
       NSString * initial = [[self lastName] substringToIndex:1];
  
       [self didAccessValueForKey:@"lastNameInitial"];
  
       return initial;
  
  }

• In your fetch request, request ONLY this PropertyDescription, like so (this is another quarter of the magic):

  NSDictionary * entityProperties = [myEntityDescription propertiesByName];
  
  NSPropertyDescription * lastNameInitialProperty = [entityProperties objectForKey:@"lastNameInitial"];
  
  [fetchRequest setPropertiesToFetch:[NSArray arrayWithObject:lastNameInitialProperty]];

• Make sure your fetch request ONLY returns distinct results (this is the last quarter of the magic):

  [fetchRequest setReturnsDistinctResults:YES];

• Order your results by this letter:
  

  NSSortDescriptor * lastNameInitialSortOrder = [[[NSSortDescriptor alloc] initWithKey:@"lastNameInitial" ascending:YES] autorelease];
  
  [fetchRequest setSortDescriptors:[NSArray arrayWithObject:lastNameInitialSortOrder]];

• execute the request, and see what it gives you.

If I understand how this works, then I'm guessing it will return an array of NSManagedObjects, each of which only has the lastNameInitial property loaded into memory, and who are a set of distinct last name initials.

Good luck, and report back on how this works. I just made this up off the top of my head and want to know if this works. =)