NSPredicate在获取请求中将@max与其他条件一起使用 [英] NSPredicate that uses @max with an other condition in a fetch request

问题描述

我有一个CoreData实体 Song ，它具有日期属性 pushDate 和一个布尔属性 isPublic .

I have a CoreData entity Song that has date property pushDate and a boolean property isPublic.

我需要获取最新的公开歌曲.

I need to fetch the latest public song.

要获取最新消息，我可以使用-

To get the latest, I can use -

request.predicate = [NSPredicate predicateWithFormat:@"pushDate==max(pushDate)"];

但是当我尝试-

request.predicate = [NSPredicate predicateWithFormat:@"pushDate==max(pushDate) AND isPublic == YES"];

我没有任何结果(我100％确信我有公开== YES的歌曲).

I don't get any result (I am 100% sure that I have songs with public == YES).

我可以使用此谓词吗?如果没有，为什么/正确的方法是什么?

Can I use this predicate ? If no why / what is the correct way to do this ?

谢谢

推荐答案

我认为问题在于 max(pushDate)是在所有歌曲中评估的，而不仅仅是公开歌曲.如果最新的歌曲(即符合第一个子句的 pushDate == max(pushDate))不是公开的，则它将使第二个子句失败( isPublic == YES ).

I think the problem is that max(pushDate) is evaluated across ALL Songs, not just the public ones. If the most recent Song (ie the one that meets the first clause, pushDate == max(pushDate)) is NOT public, then it will fail the second clause (isPublic == YES).

如果要最新的公开歌曲"，则必须先过滤掉这些公开歌曲，然后再查找最新的歌曲.

If you want "the most recent public song", then you must filter down to the public songs, before finding the most recent.

为此，我将使用仅获取公开歌曲的谓词:

To do that, I would use a predicate that fetches only public songs:

request.predicate = [NSPredicate predicateWithFormat:@"isPublic == YES"];

还要指定一个sortDescriptor，它按 pushDate 降序对结果进行排序:

And also specify a sortDescriptor that sorts the results in order of descending pushDate:

request.sortDescriptors = [[NSSortDescriptor sortDescriptorWithKey:@"pushDate" ascending:NO]];

结果中的第一项将是您想要的歌曲.为了避免获取所有行，请使用:

The first item in the results will be the song you want. To avoid fetching all the rows, use:

request.fetchLimit = 1;

执行提取之前.

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