获取NSFetchedResultsController的谓词中的属性

问题描述

我有一个Artist对象与 .localConcerts fetched属性（基本上是完整的音乐会集），我可以在NSFetchedResultsController谓词中使用该属性吗？

I have an Artist object with a .localConcerts fetched property (basically a subset of the full .concerts set), can I use that property inside my NSFetchedResultsController predicate?

这里是我想要的：

NSFetchRequest *request = [[NSFetchRequest alloc] init];

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Artist" inManagedObjectContext:context];
[request setEntity:entity];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"localConcerts.@count > 0"];
[request setPredicate:predicate];

fetchedResultsController = [[NSFetchedResultsController alloc]
                            initWithFetchRequest:request
                            managedObjectContext:context
                            sectionNameKeyPath:nil
                            cacheName:nil];

但我得到：

'keypath localConcerts not found in entity <NSSQLEntity Artist id=1>'

$ b

Am I missing anything or is it just not possible to use fetched properties inside predicates?

推荐答案

显然， NSPredicate 只能使用数据库结构中的属性进行过滤（这是有意义的）。在我的情况下，使用子查询的诀窍：

Apparently NSPredicate can only filter using attributes that are in the database structure (this makes sense). In my case, using a subquery did the trick:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"SUBQUERY(shows, $show, $show.distance < %@).@count > 0", [SWDataManager sharedManager].localFilterDistance];

我不知道我们可以做，这是伟大的知道。信用额请前往。

I had no idea we could do subqueries in NSPredicate, that's great to know. Credits go to @kyleve.

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