NSFetchRequest with distinct properties - iphone

Question

I'm trying to get a distinct result from NSPredicate.
My code:
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Members" inManagedObjectContext:context];
request.entity = entity;
request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:#"city"
ascending:YES
selector:#selector(caseInsensitiveCompare:)]];
request.predicate = [NSPredicate predicateWithFormat:#"memberDeleted == %#", [NSNumber numberWithBool:NO]];
NSDictionary *properties = [entity propertiesByName];
request.propertiesToFetch = [NSArray arrayWithObject:[properties objectForKey:#"city"]];
request.returnsDistinctResults = YES;
request.fetchBatchSize = 20;
NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:request
managedObjectContext:context
sectionNameKeyPath:nil
cacheName:#"CityCache"];
[request release];
self.fetchedResultsController = frc;
[frc release];
The issue is that the result returns many times the same City. This Entity has a lot of Members, and each Member has a property "city".
What am I doing wrong?
Thanks,
RL

Answer 1

Make sure to set the resultType of the NSFetchRequest to NSDictionaryResultType. The default is to return the actual objects, and so it will ignore propertiesToFetch.

Answer 2

With the help of #Alex, here's the final code, without FRC:
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Members" inManagedObjectContext:self.managedObjectContext];
request.entity = entity;
request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:#"city"
ascending:YES
selector:#selector(caseInsensitiveCompare:)]];
request.predicate = [NSPredicate predicateWithFormat:#"memberDeleted == %#", [NSNumber numberWithBool:NO]];
NSDictionary *properties = [entity propertiesByName];
request.propertiesToFetch = [NSArray arrayWithObject:[properties objectForKey:#"city"]];
request.returnsDistinctResults = YES;
request.resultType = NSDictionaryResultType;
request.fetchBatchSize = 20;
NSError *error = nil;
NSArray *tempArray = [self.managedObjectContext executeFetchRequest:request error:&error];
NSMutableArray *cities = [[NSMutableArray alloc] init];
for (int i=0; i < [tempArray count]; i++){
NSDictionary *tempDict = [NSDictionary dictionary];
tempDict = [tempArray objectAtIndex:i];
if ([tempDict objectForKey:#"city"] != nil)
[cities addObject:[tempDict objectForKey:#"city"]];
}
//if the tempArray has no nil values, it's more efficient with:
//NSArray* cities = [tempArray valueForKeyPath:#"city"];
self.cityArray = cities;
[cities release];
[request release];
This returns a NSArray with the list of the Cities.
Thanks.

Answer 3

From the official documentation:
NSManagedObjectContext *context = <#Get the context#>;
NSEntityDescription *entity = [NSEntityDescription entityForName:#"<#Entity name#>" inManagedObjectContext:context];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];
[request setResultType:NSDictionaryResultType];
[request setReturnsDistinctResults:YES];
[request setPropertiesToFetch :[NSArray arrayWithObject:#"<#Attribute name#>"]];
// Execute the fetch.
NSError *error;
id requestedValue = nil;
NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error];
if (objects == nil) {
// Handle the error.
}