与 NSfetchedresultscontroller 的一对多关系

Posted 2023-03-03

【中文标题】与 NSfetchedresultscontroller 的一对多关系

【英文标题】：One to many relationship with NSfetchedresultscontroller

【发布时间】：2016-07-21 12:21:43

【问题描述】：

我在从一对多关系中获取对象时遇到问题。 我有两个实体“Featured”和“Event”，Featured 对象包含 featuresEvents 的属性，这是一对多关系，一个“Featured”项目到多个“Events”。

现在我需要使用 NSFetchedResultsController 来返回“featuredEvents”关系中的事件。我正在使用以下代码：

 NSFetchedResultsController *fetchedFeaturedEvents;
    NSFetchRequest *fetchRequest;
    NSError *error = nil;

    fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"SDMFeaturedEvent"];
    [fetchRequest setSortDescriptors:@[]];

    fetchedFeaturedEvents = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:theManagedObjectContext sectionNameKeyPath:nil cacheName:nil];
    [fetchedFeaturedEvents performFetch:&error];

    if (error) 
        NSLog(@"Unable to perform fetch.");
        NSLog(@"%@, %@", error, error.localizedDescription);
    

    return fetchedFeaturedEvents;

这很好，我返回了特色事件，但我还希望添加一个谓词以仅返回“featuredEvents”属性中的事件。所以问题是我如何创建谓词？

我基本上需要“精选”对象的 featuresEvents 属性中的所有“事件”，这可能吗？

【问题讨论】：

featuredEvents 属性已经为您提供了该功能的所有事件。访问该属性会自动导致故障触发并加载事件 - 无需查询事件。不确定我是否理解您的问题。

是的，我可以通过这种方式访问​​它们，但是是否可以通过谓词访问它们，因为我想在它们上运行其他谓词方法？

创建一个过滤特征反向关系的谓词。对于语法，请查看通过 Google 获得的无数示例和操作方法。

顺便说一句，您确实应该有一个排序描述符，否则 FRC 将在上下文更新时确定更改时遇到问题。

嗨，Avi，我试过这个：[fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"SDMEvent IN featuresEvents"]];但是我遇到了以下问题：NSInvalidArgument（LHS 和 RHS 都是关键路径）一旦我让它工作，我会给它一个排序描述符。

【参考方案1】：

对于那些一直有类似问题的人，我最终使用了这种方法：

NSFetchRequest *fetchRequest;
    NSError *error = nil;
    NSArray *fetchedObjects;
    NSPredicate *soldOutPredicate;
    NSPredicate *criteriaMatchPredicate;
    NSCompoundPredicate *fullPredicate;
    NSSortDescriptor *usesDescriptor = [[NSSortDescriptor alloc] initWithKey:@"eventDate" ascending:YES];

    fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"FeaturedEvent"];
    [fetchRequest setSortDescriptors:@[]];
    fetchedObjects = [theManagedObjectContext executeFetchRequest:fetchRequest error:&error];

    if (error) 
        NSLog(@"Unable to perform fetch.");
        NSLog(@"%@, %@", error, error.localizedDescription);
    

    if ([[NSSDMUserDetails sharedManager]showsCriteraMatchEvents]) 

        if ([[NSSDMUserDetails sharedManager]isUserMale]) 
            criteriaMatchPredicate = [NSPredicate predicateWithFormat:@"(eventAgeMaleMin <= %@) AND (eventAgeMaleMax >= %@)",[[[NSSUserDetails sharedManager]userAge]stringValue],[[[NSUserDetails sharedManager]userAge]stringValue]];
        
        else
            criteriaMatchPredicate = [NSPredicate predicateWithFormat:@"(eventAgeFemaleMin <= %@) AND (eventAgeFemaleMax >= %@)",[[[NSUserDetails sharedManager]userAge]stringValue],[[[NSUserDetails sharedManager]userAge]stringValue]];
        

        fullPredicate = [NSCompoundPredicate andPredicateWithSubpredicates:@[criteriaMatchPredicate]];

    

    if (![[NSSDMUserDetails sharedManager]showsSoldOutEvents]) 

        if ([[NSSDMUserDetails sharedManager]isUserMale]) 
            soldOutPredicate = [NSPredicate predicateWithFormat:@"eventPlacesMale == %@ || eventPlacesMale != %@",@"in stock",@"0"];
        
        else
            soldOutPredicate = [NSPredicate predicateWithFormat:@"eventPlacesFemale == %@ || eventPlacesFemale != %@",@"in stock",@"0"];
        

        fullPredicate = [NSCompoundPredicate andPredicateWithSubpredicates:@[soldOutPredicate]];
    

    if (soldOutPredicate && criteriaMatchPredicate) 

        fullPredicate = [NSCompoundPredicate andPredicateWithSubpredicates:@[soldOutPredicate,criteriaMatchPredicate]];

    

    if (fetchedObjects && fetchedObjects.count > 0) 

        SDMFeaturedEvent *feat = [fetchedObjects objectAtIndex:0];
        NSLog(@"All featured events %@",feat.featuredEvents);
        if (fullPredicate) 
            return [[feat.featuredEvents.allObjects filteredArrayUsingPredicate:fullPredicate]sortedArrayUsingDescriptors:@[usesDescriptor]];
        
        else if (![[NSSDMUserDetails sharedManager]showsCriteraMatchEvents])
            return [feat.featuredEvents.allObjects sortedArrayUsingDescriptors:@[usesDescriptor]];
        
        else
            return [NSArray new];
        

    
    return [NSArray new];

诀窍是从featuredEvent“featuredEvents”属性返回一个事件数组，然后您可以运行filteredArrayUsingPredicate 和sortedArrayUsingDescriptor 方法来根据您的要求对数组中的事件对象进行排序。当然，这不会返回 NSFetchedResultsController ，但这对于过滤非常有用。