如何为一对多关系正确设置 NSPredicate 以检查存在对象

Question

我有两个表 TRIP 和 ALERT

关系是一对多 一次行程可以包含 1 个或多个警报， 我想获取那些包含警报的行程

我提出这样的获取请求，请资产我亲爱的

-(NSFetchRequest*) fetchRequestGetTriphaveAlert{ NSFetchRequest *fetchRequest = [[NSFetchRequest alloc]init];

NSEntityDescription *trip = [NSEntityDescription entityForName:@"Trip" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:trip];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"alert == %@",!NULL];
[fetchRequest setPredicate:predicate];
NSSortDescriptor *sort = [NSSortDescriptor sortDescriptorWithKey:@"startDate" ascending:YES];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];
return fetchRequest;

}

Answer 1

试试这个：

NSPrediate *predicate = [NSPredicate predicateWithFormat:@"alert.@count > 0"];