如何对Core Data获取的属性进行排序

Question

该核心数据文件指出:

与[fetched]属性关联的获取请求可以具有排序顺序,因此可以对所获取的属性进行排序.

如何在Xcode的数据模型编辑器中为fetched属性指定排序描述符？我无法在任何地方找到相关领域.我正在为iPhone平台开发,如果这有任何区别.

如果通过图形模型编辑器无法实现这一点,那么如何在代码中修改fetched属性的获取请求以使其具有排序描述符？

Answer 1

您实际上可以获取模型获取属性并向其添加排序描述符(同样,在代码中).如果您选择其中一个带有Core Data的模板,我会在您的AppDelegate中生成的标准方法中执行此操作:

顺便说说.这会对数据模型中所有模型上的所有获取属性进行排序.您可以使用它获得花哨和适应性,但它​​是处理7个单独模型的最简洁方法,每个模型都获取了需要按名称排序的属性.效果很好.

/**
 Returns the managed object model for the application.
 If the model doesn't already exist, it is created by merging all of the models found in the application bundle.
 */
- (NSManagedObjectModel *)managedObjectModel {

    if (managedObjectModel != nil) {
        return managedObjectModel;
    }
    managedObjectModel = [[NSManagedObjectModel mergedModelFromBundles:nil] retain];    

    // Find the fetched properties, and make them sorted...
    for (NSEntityDescription *entity in [managedObjectModel entities]) {
        for (NSPropertyDescription *property in [entity properties]) {
            if ([property isKindOfClass:[NSFetchedPropertyDescription class]]) {
                NSFetchedPropertyDescription *fetchedProperty = (NSFetchedPropertyDescription *)property;
                NSFetchRequest *fetchRequest = [fetchedProperty fetchRequest];

                // Only sort by name if the destination entity actually has a "name" field
                if ([[[[fetchRequest entity] propertiesByName] allKeys] containsObject:@"name"]) {
                    NSSortDescriptor *sortByName = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
                    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortByName]];
                    [sortByName release];
                }
            }
        }
    }

    return managedObjectModel;
}

Answer 2

您没有在图形编辑器中指定它们(据我所知).

您可以在进行提取的代码中指定它们.

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

// here's where you specify the sort
NSSortDescriptor* sortDescriptor = [[NSSortDescriptor alloc]
                                initWithKey:@"name" ascending:YES];
NSArray* sortDescriptors = [[[NSArray alloc] initWithObjects: sortDescriptor, nil] autorelease];
[request setSortDescriptors:sortDescriptors];
[sortDescriptor release];

fetchedResultsController = [[NSFetchedResultsController alloc]
               initWithFetchRequest:request
               managedObjectContext:self.managedObjectContext
                 sectionNameKeyPath:nil
                          cacheName:@"myCache"];

Answer 3

建模工具似乎没有办法在获取请求上设置排序描述符.

在加载模型之后但在将其与持久性存储协调器关联之前,应该可以[1]找到要为其控制排序顺序的已获取属性描述,并将其获取请求替换为具有排序的获取请求设置描述符.

[1]原则上这应该有效.在实践中,我还没有这样做或测试过它.

Answer 4

使用Tim Shadel的好答案我添加了per-NSManagedObject子类排序...

...在Tier.m(这是一个NSManagedObject子类)...

+ (void)initialize
{
    if(self == [Tier class])
    {
        NSFetchedPropertyDescription *displayLessonPropertyDescription = [[[Tier entityDescription] propertiesByName] objectForKey:@"displayLesson"];
        NSFetchRequest *fetchRequest = [displayLessonPropertyDescription fetchRequest];

        NSSortDescriptor *sortByName = [[NSSortDescriptor alloc] initWithKey:@"displayOrder" ascending:YES];
       [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortByName]];
        [sortByName release];
    }
}