如何在核心数据中应用group by子句

我正在实现 tableview，我想在 tableview 部分显示类名，并且我正在尝试获取 使用核心数据实现的数据库中的类值，我想使用类名上的 group by 子句获取数据，这是我的代码

   AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication]delegate];
    //create managed object context
    NSManagedObjectContext *context = [appDelegate managedObjectContext];


    NSFetchRequest* fetch = [NSFetchRequest fetchRequestWithEntityName:@"Boking_Detail"];
    NSEntityDescription* entity = [NSEntityDescription entityForName:@"Boking_Detail"
                                              inManagedObjectContext:context];
    NSAttributeDescription* clsName = [entity.attributesByName objectForKey:@"classname"];
    NSAttributeDescription* clsDate = [entity.attributesByName objectForKey:@"classdate"];
    NSAttributeDescription* startTime = [entity.attributesByName objectForKey:@"starttime"];
    NSAttributeDescription* endTime = [entity.attributesByName objectForKey:@"endtime"];

    NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
    [expressionDescription setName: @"count"];
   // [expressionDescription setExpression: countExpression];
    [expressionDescription setExpressionResultType: NSInteger32AttributeType];
    [fetch setPropertiesToFetch:[NSArray arrayWithObjects:clsName,clsDate,startTime,endTime, expressionDescription, nil]];
    [fetch setPropertiesToGroupBy:[NSArray arrayWithObject:clsName]];
    [fetch setResultType:NSDictionaryResultType];
    NSError* error = nil;
    NSArray *results = [context executeFetchRequest:fetch  error:&error];

    NSLog(@"result array count %i",results.count);
     NSLog(@"result array values %@",results);

我收到这个错误

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'SELECT clauses in queries with GROUP BY components can only contain properties named in the GROUP BY or aggregate functions ((<NSAttributeDescription: 0xc288680>), name classdate, isOptional 1, isTransient 0, entity Boking_Detail, renamingIdentifier classdate, validation predicates (
), warnings (
), versionHashModifier (null)
 userInfo {
}, attributeType 700 , attributeValueClassName NSString, defaultValue (null) is not in the GROUP BY)'
*** First throw call stack:

您还必须添加其他属性propertiesToFetch到 group by 子句，但表达式除外：

[fetch setPropertiesToGroupBy:[NSArray arrayWithObjects:clsName, clsDate, startTime, endTime, nil]];