Question

我遇到从一对多关系中获取对象的问题。我有两个实体“精选”和“事件”，特色对象包含一个特色事件的属性，这是一个多人关系，一个“特色”项目到很多“事件”。

现在我需要使用NSFetchedResultsController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;

这很棒我返回特色事件，但我也想添加一个谓词，只返回“featuredEvents”属性中的事件。所以问题是如何创建谓词？

我基本上需要所有“事件”在“特色”对象的featuredEvents属性中进行提取，这可能吗？

Answer 1

对于那些遇到类似问题的人，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featuresEvent“featuredEvents”属性返回一系列事件，然后您可以运行filteredArrayUsingPredicate和sortedArrayUsingDescriptor方法，以根据您的要求对数组中的事件对象进行排序。虽然这不是返回NSFetchedResultsController，但这适用于过滤。

与NSfetchedresultscontroller的一对多关系

1 个答案: