setPropertiesToFetch 返回“无效的键路径表达式”错误

Question

我正在尝试以最基本的形式使用 group-by 核心数据功能：

- (NSFetchedResultsController *)fetchedResultsController {

    NSManagedObjectContext *managedObjectContext = [self managedObjectContext];

    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSExpression *reviewNumKeyExpression = [NSExpression expressionForKeyPath:@"review_num"];
    NSExpression *reviewMaxExpression = [NSExpression expressionForFunction:@"max:" arguments:[NSArray arrayWithObject:reviewNumKeyExpression]];

    NSExpressionDescription *maxReviewED = [[NSExpressionDescription alloc] init];
    [maxReviewED setName:@"maxReview"];
    [maxReviewED setExpression:reviewMaxExpression];
    [maxReviewED setExpressionResultType:NSInteger32AttributeType];

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
                                   entityForName:@"Group" inManagedObjectContext:managedObjectContext];

    [fetchRequest setResultType:NSDictionaryResultType];
    [fetchRequest setReturnsDistinctResults:YES];
    [fetchRequest setEntity:entity];

    NSAttributeDescription *groupId = [entity.propertiesByName objectForKey:@"group_id"];
    NSArray *propertiesToFetch = [NSArray arrayWithObjects:groupId, maxReviewED, nil];
    [fetchRequest setPropertiesToFetch:propertiesToFetch];
    [fetchRequest setPropertiesToGroupBy:propertiesToFetch];
...

运行此代码时，我收到以下错误：

由于未捕获的异常而终止应用程序 'NSInvalidArgumentException'，原因：'无效的键路径表达式 ((), 名称 maxReview, isOptional 1, isTransient 0, entity (null), renamingIdentifier maxDepartment、验证谓词 ()、警告 ()、 versionHashModifier (null) userInfo { }) 传递给 setPropertiesToFetch

我想我在 API 上遗漏了一些东西，所以非常感谢任何帮助！

Answer 1

我认为错误信息是错误的：问题出在setPropertiesToGroupBy，而不是setPropertiesToFetch。您不能按计算值 (maxReview) 分组，但我怀疑您是否真的想要：如果您想要每个 groupId 的最大值 review_num，您只需要 groupId 中的 propertiesToGroupBy:

NSAttributeDescription *groupId = [entity.propertiesByName objectForKey:@"group_id"];
NSArray *propertiesToFetch = [NSArray arrayWithObjects:groupId, maxReviewED, nil];
NSArray *propertiesToGroupBy = [NSArray arrayWithObjects:groupId, nil];
[fetchRequest setPropertiesToFetch:propertiesToFetch];
[fetchRequest setPropertiesToGroupBy:propertiesToGroupBy];