Question

我正在尝试提取属于最新Actions的所有Session。

在SQL中，这将是一个简单的JOIN ... WHERE Session.startTime = MAX（Session.startTime），但是使用Core Data我无法弄清楚如何在不采取以下两个步骤的情况下执行此操作。有什么想法吗？

// Step 1. Get the latest session

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@"Session" inManagedObjectContext:self.managedObjectContext]];
[request setSortDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"startTime" ascending:NO]]];
[request setFetchLimit:1];

Session *lastSession = nil;

NSArray *objects = [self.managedObjectContext executeFetchRequest:request error:NULL]; 

if ([objects count] > 0)
{ 
    NSLog(@"max: %@", [objects objectAtIndex:0]);
    lastSession = [objects objectAtIndex:0];
}

// Step 2. Get that session's actions

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Action" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];

NSSortDescriptor *sectionSort = [[NSSortDescriptor alloc] initWithKey:@"session.startTime" ascending:NO selector:nil];
NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"startTime" ascending:NO selector:nil];
[fetchRequest setSortDescriptors:[NSArray arrayWithObjects:sectionSort, sort, nil]];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"session.startTime = %@", lastSession.startTime];
//**Can't I do something like this instead?**
//NSPredicate *predicate = [NSPredicate predicateWithFormat:@"session.startTime = session.@max.startTime"];
[fetchRequest setPredicate:predicate];

[fetchRequest setFetchBatchSize:10];

[self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"session.startTime" cacheName:@"ActionHistory"];
self.fetchedResultsController.delegate = self;

NSError *error;
if (![self.fetchedResultsController performFetch:&error])
{
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();  // Fail
}

编辑：我确实需要坚持使用上面没有详细说明的NSFetchedResultsController正当理由。

Answer 1

我建议你与Session和Action建立关系。然后，在您获取所需的会话后，您可以将所有会话的操作视为简单的lastSession.actions。

修改也许你可以做到

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"session = %@",lastSession];

NSFetchRequest，可以通过聚合函数进行过滤吗？

1 个答案: