iOS:对 NSFetchedResultsController 进行排序和分组 [英] iOS: Sorting and grouping NSFetchedResultsController

问题描述

我有两个具有以下结构的表

I have two tables with the following structure

主要类别:

name position hasSubCategories

子类别:

name position display belongsToMainCategory

现在我想显示按主类别分组的所有子类别(其中显示属性 = YES).主要类别部分应按位置定义排序，子类别本身(在部分内)按其位置属性进行排序.(顺便说一下，对于某个主要类别，名称可以相同……我的表具有更多属性，但它们与理解问题无关).但我的订单完全搞砸了.为什么?这是我的 FetchedResultsController 代码:

Now I want to display all subcategories (where display attribute = YES) grouped by main category. The main category sections should be sorted as defined by position and the subcategories itself (within the section) by their position attribute. (name by the way can be the same for a certain main category...my tables have more attributes but they aren't relevant to understand the problem). But my order is completely messed up. Why? Here's my code for the FetchedResultsController:

    NSError *error = nil;
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"SubCategory"];

    NSSortDescriptor *mainCatPosition = [[NSSortDescriptor alloc]
                            initWithKey:@"belongsToMainCategory.position" ascending:YES];
    NSSortDescriptor *subCatPosition = [[NSSortDescriptor alloc]
                            initWithKey:@"position" ascending:YES];

    request.sortDescriptors = [NSArray arrayWithObjects:mainCatPosition,subCatPosition,nil];
    request.predicate = [NSPredicate predicateWithFormat:@"display = %@", [NSNumber numberWithBool:YES]];
    [self.db.managedObjectContext executeFetchRequest:request error:&error];

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

推荐答案

用作 sectionNameKeyPath: 参数的关键路径到获取的结果控制器必须与第一个排序描述符中使用的键相同或生成相同的相对顺序.

The key path used as sectionNameKeyPath: argument to the fetched results controller must be the same key that is used in the first sort descriptor or generate the same relative ordering.

fetched results controller首先根据第一个对所有fetched对象进行排序对描述符进行排序，然后根据 sectionNameKeyPath 将对象分组.因此，使用不同的键路径(例如belongsToMainCategory.position"与belongsToMainCategory.name")不起作用.这甚至可能导致有关乱序部分"的运行时错误.

The fetched results controller first orders all fetched objects according to the first sort descriptor and then groups the objects into sections according to the sectionNameKeyPath. Therefore using different key paths (as in your case "belongsToMainCategory.position" vs. "belongsToMainCategory.name") does not work. This could even cause a runtime error about "out of order sections".

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