NSFetchRequest with distinct properties

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

score 7 · Accepted Answer · answered Oct 08 '11 at 18:48

7

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.

answered Oct 08 '11 at 18:48

Alex

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I should probably add that `NSFetchedResultsController` is not generally a fan of the `NSDictionaryResultType`. You may run into other problems and have to abandon the `NSFetchedResultsController`. – Alex Oct 08 '11 at 18:57
The question is that I don't want the result to be and NSDictionary. If not, I'll have to change some of my code. Is it doable other way? – Rui Lopes Oct 08 '11 at 19:45
What object are you expecting it to return, then? Core Data can only return objects that actually exist in your object graph. – Alex Oct 08 '11 at 19:51
I've got it. You are correct. I was thinking in getting the Members, and group them by city. I do not have a City object, so your suggestion is correct. My thought was wrong. Not use the NSFetchedResultsController is the best choice. Can you update the answer, so that other see it better? – Rui Lopes Oct 08 '11 at 19:54

Rui Lopes · Answer 2 · 2011-10-09T13:31:18.113

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.

You can do this more easily. Just do this: `NSArray* cities = [tempArray valueForKeyPath:@"city"];` This will get the value of `city` for each dictionary in the array and put them in an array. — Alex, Oct 08 '11 at 21:26
@Alex, that works fine if there's no nil value in the Array. If there is any nil value, it crash. — Rui Lopes, Oct 09 '11 at 13:29
Oops. I upvoted Rui's "crash" comment while investigating this issue, thinking it a valuable caution. But the NSArray class ref lists a slightly different method, "valueForKey:", which it says will return an NSNull object for nil objects. So there shouldn't be a problem. — Wienke, Feb 18 '12 at 16:30

score 3 · Answer 3 · answered Jun 28 '12 at 20:19

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.
}

NSFetchRequest with distinct properties

3 Answers3