根据对多关系中相关对象的计数对获取请求进行排序答案

【问题标题】：Sorting a fetch request based on count of related object in to-many relationship根据对多关系中相关对象的计数对获取请求进行排序
【发布时间】：2015-12-14 00:04:59
【问题描述】：

需要按类别拥有的产品数量/计数对通过 fetch 请求检索到的类别列表进行排序。然后，我希望能够手动将几个类别（被过滤掉）添加到数组中，以便它们位于列表的底部。当我尝试按产品数量进行排序时，我一直遇到“包含 KVC 聚合的 Keypath 不应该有...”的错误。

我在这里错过了什么/做错了什么？谢谢！

类别.h

@class Category;

@interface Category : NSManagedObject

@property (nonatomic, strong) NSString *name;
@property (nonatomic, strong) NSSet *products;

@end

产品.h

@class Category;

@interface Product : NSManagedObject

@property (nonatomic, strong) NSString *sku;
@property (nonatomic, strong) NSString *name; // Product name
@property (nonatomic, strong) NSString *prodDescription;
@property (nonatomic, strong) Category *category; // Product category
@property (nonatomic, strong) NSString *upc;
@property (nonatomic, strong) NSString *countryCode;
@property (nonatomic, strong) NSString *webpage;
@property (nonatomic, strong) NSString *manual;
@property (nonatomic, strong) NSString *quickStart;
@property (nonatomic, strong) UIImage *thumb;
@property (nonatomic, strong) NSString *thumbURLString;
@property (nonatomic, strong) UIImage *mainImg;
@property (nonatomic, strong) NSString *mainImgURLString;
@property (nonatomic, strong) UIImage *secondaryImg;
@property (nonatomic, strong) NSString *secondaryImgURLString;

@end

CategoriesViewController.m

- (NSFetchedResultsController *)fetchedResultsController {

    if (_fetchedResultsController == nil) {
        NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
        [fetchRequest setEntity:[NSEntityDescription entityForName:@"Category" inManagedObjectContext:self.managedObjectContext]]; // Fetch categories
        NSArray *sortDescriptors = nil;
        NSString *sectionNameKeyPath = nil;
        // Sort by number of products in category
        sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"category.@count" ascending:NO]];
        [fetchRequest setSortDescriptors:sortDescriptors];

        // Use predicate to exclude Accessories and Replacement Parts categories from initial array
        NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name != 'Accessories' AND name != 'Replacement Parts'"];
        [fetchRequest setPredicate:predicate];

        _fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                                                    managedObjectContext:self.managedObjectContext
                                                                      sectionNameKeyPath:sectionNameKeyPath
                                                                               cacheName:nil];

        }
    return _fetchedResultsController;
}

【问题讨论】：

标签： ios objective-c core-data

【解决方案1】：

你应该使用这样的一对多关系

【讨论】：

【解决方案2】：

通常，这可以通过在记录本身中添加一个计数变量来解决。这允许获取的结果控制器对该计数的值进行排序。我在相关关系的自定义设置器中维护这种计数。在您的情况下，这可能是products 的集合，新变量可能称为productCount。 Xcode 为这类事情提供了一个 sn-p 模板。这是一个例子：

- (void) addProductsObject: (Product *) value {

    NSSet *changedObjects = [NSSet setWithObject:value];

    [self willChangeValueForKey: kDays withSetMutation: NSKeyValueUnionSetMutation usingObjects: changedObjects];
    [[self primitiveValueForKey: kDays] addObject: value];
    [self  didChangeValueForKey: kDays withSetMutation: NSKeyValueUnionSetMutation usingObjects: changedObjects];

    self.productCount = (int32_t)self.products.count;

} // -addProductsObject:

其他三种方法同样简单。

【讨论】：