Actually it’s pretty simple to do. But if you try just by reading the documentation you’ll find the same problem I did.

Suppose that I need to get a list of all ages I had on my database to make a hierarchical navigation that starts by selecting an age. If were working directly with the SQLite store I could execute this SQL to do this:

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SELECT DISTINCT AGE
FROM PEOPLE
ORDER BY AGE

Since the project uses Core Data I shouldn’t access the underlying database store directly. Use NSFetchRequest class instead of plain SQL. The returnsDistinctsResults property seems to do the trick, its documentation warns about a special consideration:

Returns a Boolean value that indicates whether the fetch request returns only distinct values for the fields specified by propertiesToFetch.

Special Considerations

This value is only used if a value has been set for propertiesToFetch.

So it’s saying is that the returnsDistinctsResults only works if the property propertiesToFetch is also set. Pretty straightforward. Except that there’s one more - undocumented - property that needs to be set: resultType.

The default value of resultType is NSManagedObjectResultType which returns the expected managed objects. To make the distinct results works the value needs to be set to NSDictionaryResultType.

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NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"People" inManagedObjectContext:managedObjectContext];
request.entity = entity;
request.propertiesToFetch = [NSArray arrayWithObject:[[entity propertiesByName] objectForKey:@"age"]];
request.returnsDistinctResults = YES;
request.resultType = NSDictionaryResultType;

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"age" ascending:YES];
[request setSortDescriptors:[NSArray arrayWithObject:sortDescriptors]];
[sortDescriptor release];

NSError *error = nil;
NSArray *distincResults = [managedObjectContext executeFetchRequest:request error:&error];
// Use the results
[request release];

If you forget to set the resultType there will be no errors, but the returnsDistinctResults will be ignored and your result will have all ages of the database.