Count Distinct items in Core Data - iphone

Question

I have a Core Data entity with a property named 'value' which is often repeated. I wish to retrieve only unique values (done) and also how often each one appears, so that I can sort by that property (I'm building an autocomplete function based on existing user input, so knowing how frequently a certain input has appeared is essential).
My fetch request currently looks like this:
NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Item" inManagedObjectContext:self.managedObjectContext];
NSDictionary *entityProperties = [entity propertiesByName];
[fetchRequest setEntity:entity];
[fetchRequest setFetchBatchSize:10];
[fetchRequest setFetchLimit:20];
[fetchRequest setReturnsDistinctResults:YES];
[fetchRequest setResultType:NSDictionaryResultType];
[fetchRequest setPropertiesToFetch:[NSArray arrayWithObject:[entityProperties objectForKey:#"value"]]];
NSSortDescriptor *sortDescriptor = [[[NSSortDescriptor alloc] initWithKey:#"value" ascending:YES] autorelease];
NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"value BEGINSWITH[c] %#",predicateString];
[fetchRequest setPredicate:predicate];
return fetchRequest;
I'm a bit stuck with this one. Any ideas?
Thanks in advance!

Answer 1

I don't like this one, but...
You could do an other fetchRequest with a predicate that exactly match the value you are seeking. Allowing duplicate, then have the count of the array.
This one is better, but more work up front.
An other way could be to have a derived property in your data model that keep track of your duplicated count as you create them.
(with that option you could easily sort by duplicated count)
Ok for a Derived property.
First you will need to subclass NSManagedObject and use that subclass in your data model. (in Xcode 3 there was a way to create that quickly, but I don't know that in Xcode 4) But if you name it the same as your entity I think core data will pick it up.
#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>
#interface Person : NSManagedObject
#property (nonatomic, retain) NSString * name;
#property (nonatomic, retain) NSString * firstLetterOfName;
#property (nonatomic, retain) NSString * phoneNumber;
#end
And in your implementation you will need to do something like this (there is code snippet in Xcode 4 to get you started, but there is a typo in one of them, unless it have been corrected)
#import "Person.h"
#implementation Person
#dynamic name, phoneNumber, firstLetterOfName;
- (void)setName:(NSString *)value {
[self willChangeValueForKey:#"name"];
[self setPrimitiveValue:value forKey:#"name"];
self.firstLetterOfName = [value substringToIndex:1];
[self didChangeValueForKey:#"name"];
}
#end
You can see that the firstLetterOfName is set each time the Name is set.
You can do the same kind of thing with relationship.
So when you add an item to a relationship you should be able to look up your relationship.
Here is an exemple of something similar, where I need to find if the object I'm adding have the lowest price of it's group because of a derived property call isMeilleurPrixAvecPrixElment. (this is old code, so I don't recall every detail of it, it have been done in OSX.4)
- (void)addPrixHistoriqueObject:(PrixElement_MO *)value
{
NSSet *changedObjects = [[NSSet alloc] initWithObjects:&value count:1];
[self willChangeValueForKey:#"prixHistorique" withSetMutation:NSKeyValueUnionSetMutation usingObjects:changedObjects];
if ([self isPrixRegulierAvecPrixElement:value])
[self enleveL_AutrePrixRegulierPourCommerceDeCePrixElement:value];
if ([self isMeilleurPrixAvecPrixElment:value])
[self echangeMeilleurPrixAvecCePrixElement:value];
[[self primitivePrixHistorique] addObject:value];
[self didChangeValueForKey:#"prixHistorique" withSetMutation:NSKeyValueUnionSetMutation usingObjects:changedObjects];
[changedObjects release];
}
In respond to a comment
depending on your data model and the importance of that aspect in your application, I can think of 3 solutions.
1- Redesing your data model around that aspect.
2- When setting the values query the rest of your entity with a predicate and update a property that have the count.
3- (I'm not sure of that one, but is worth trying) NSManagedObect is an object, so maybe you could have a static dictionary that have the value as a key and the count as value.
I would maybe try number 3 first (it look like the easy one), but I've never done something like that. So I'm not sure for the presitance of a class variable in core data.