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我在 iOS-CoreData 上下文中创建有效的 NSPredicate 时遇到问题。该表包含所有好友的所有消息。我需要得到每个好友的最新消息：

我尝试过使用 DBVisualizer，我需要以下 SQL（工作正常）

    /* for testing with a db tool like DbVisualizer
     select ZBAREJIDSTR,ZBODY, ZTIMESTAMP
     from ZXMPPMESSAGEARCHIVING_MESSAGE_COREDATAOBJECT
     group by ZBAREJIDSTR
     having max(ZTIMESTAMP)
     order by ZTIMESTAMP desc;
     */

以下代码因“无法解析格式字符串”而失败，我找不到适合我的示例。

    - (NSFetchedResultsController *)fetchedResultsController {
if (fetchedResultsController == nil)    {

    NSManagedObjectContext *moc = [[Utilities chatManagerFromAppDelegate] managedObjectContext_messageArchiving];

    NSEntityDescription *entity = [NSEntityDescription entityForName:@"XMPPMessageArchiving_Message_CoreDataObject"
                                              inManagedObjectContext:moc];

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSAttributeDescription* bodyDesc = [entity.attributesByName objectForKey:@"body"];
    NSAttributeDescription* barJidStrDesc = [entity.attributesByName objectForKey:@"barJidStr"];
    NSAttributeDescription* timestampDesc = [entity.attributesByName objectForKey:@"timestamp"];

    NSPredicate *predicateMaxTimestamp = [NSPredicate predicateWithFormat:@"@max.timestamp"];

    NSSortDescriptor *sortDescriptorTimestampDesc = [[NSSortDescriptor alloc] initWithKey:@"timestamp" ascending:NO];
    NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptorTimestampDesc, nil];

    [fetchRequest setEntity:entity];
    [fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:barJidStrDesc, bodyDesc, timestampDesc, nil]];
    [fetchRequest setPropertiesToGroupBy:[NSArray arrayWithObject:bodyDesc]];
    [fetchRequest setHavingPredicate: predicateMaxTimestamp];
    [fetchRequest setSortDescriptors:sortDescriptors];
    [fetchRequest setResultType:NSDictionaryResultType];
    [fetchRequest setFetchBatchSize:10];
    fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                                                   managedObjectContext:moc
                                                                     sectionNameKeyPath:nil
                                                                              cacheName:nil];
    ....

实际上我也很欣赏其他替代方案，因为我需要更新底层模型（[fetchedResultsController setDelegate:self];），它不适用于 NSDictionaryResultType（但 NSDictionaryResultType 对于 group by 是必需的）。

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1 回答 1

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首先，如果你想“分组”，你需要指定获取的结果控制器sectionNameKeyPath应该是“barJidStr”。

其次，最大时间戳谓词格式字符串的正确语法是
@"ANY @max.timestamp".

第三，您的 SQL 和属性名称之间存在差异。如果您的属性定义为“barJidStr”，则原始 SQL 应该是“ZBARJIDSTR”，而不是“ZBAREJIDSTR”。

于 2013-01-09T23:16:14.647 回答