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ioscore-datansfetchrequest

CoreData get distinct values of Attribute

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?

Solution

  • 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);