尝试按日期分组核心数据获取结果

Question

I want to group core data entries by week and date. I have a core data entity named WorkEntry which among it's attributes I have weekBegin and date . In these I save the date it was saved on as well as the first day of the week corresponding to that date. I use weekBegin for the section name and I want the cells to show the dates that have entries with no duplicates. I used http://felipecypriano.com/2011/09/21/core-data-how-to-do-a-select-distinct/ as a guide. Here's my code:

Fetch Request with returnsDistinctResults :

- (NSFetchRequest *)workDayFetchRequest {
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"WorkEntry"];
    CoreDataStack *coreDataStack = [CoreDataStack defaultStack];

    NSEntityDescription *entity = [NSEntityDescription entityForName:@"WorkEntry" inManagedObjectContext:coreDataStack.managedObjectContext];
    fetchRequest.entity = entity;
    fetchRequest.propertiesToFetch = [NSArray arrayWithObject:[[entity propertiesByName] objectForKey:@"weekBegin"]];
    fetchRequest.returnsDistinctResults = YES;
    fetchRequest.resultType = NSDictionaryResultType;

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"weekBegin" ascending:YES];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];

    NSError *error = nil;
    self.distinctResults = [coreDataStack.managedObjectContext executeFetchRequest:fetchRequest error:&error];

    return fetchRequest;
}

which gives me an array called distinctResults which I use in:

CellForRowAtIndexPath :

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];

    cell.textLabel.text = [self.distinctResults objectAtIndex:indexPath.row];

    return cell;
}

But I have a feeling that may not be right. When I switch to the table view in the app it crashes with this exception:

-[NSKnownKeysDictionary1 length]: unrecognized selector sent to instance 0x8c7c8a0

I'd be really grateful for any help you guys can give. I've been going crazy trying to figure this out. Thanks

Answer 1

The problem may be in the way you declare distinctResults property in you controller. It may be deallocated if you use weak pointer.