CoreData get distinct values of Attribute

ephilip picture ephilip · Aug 9, 2011 · Viewed 29.5k times · Source

I'm trying to setup my NSFetchRequest to core data to retrieve the unique values for a specific attribute in an entity. i.e.

an entity with the following information:

  name | rate | factor |
_______|______|________|
John   |  3.2 |    4   |
Betty  |  5.5 |    7   |
Betty  |  2.1 |    2   |
Betty  |  3.1 |    2   |
Edward |  4.5 |    5   |
John   |  2.3 |    4   |

How would i set up the request to return an array with just: John, Betty, Edward?

Answer

AlleyGator picture AlleyGator · Jul 9, 2012

You should use the backing store to help you get distinct records.

If you want to get an array with just John, Betty, Edward here's how you do it:

NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"MyEntity"];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"MyEntity" inManagedObjectContext:self.managedObjectContext];

// Required! Unless you set the resultType to NSDictionaryResultType, distinct can't work. 
// All objects in the backing store are implicitly distinct, but two dictionaries can be duplicates.
// Since you only want distinct names, only ask for the 'name' property.
fetchRequest.resultType = NSDictionaryResultType;
fetchRequest.propertiesToFetch = [NSArray arrayWithObject:[[entity propertiesByName] objectForKey:@"name"]];
fetchRequest.returnsDistinctResults = YES;

// Now it should yield an NSArray of distinct values in dictionaries.
NSArray *dictionaries = [self.managedObjectContext executeFetchRequest:fetchRequest error:nil];
NSLog (@"names: %@",dictionaries);