如何使用 MagicalRecord 过滤和获取记录？

Question

我有两个这样的类，

@class Songs;

@interface Album : NSManagedObject

@property (nonatomic, retain) NSNumber * albumID;
@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSSet *songs;

@end

@interface Album (CoreDataGeneratedAccessors)
- (void)addSongsObject:(Songs *)value;
- (void)removeSongsObject:(Songs *)value;
- (void)addSongs:(NSSet *)values;
- (void)removeSongs:(NSSet *)values;
@end
@class Album;

@interface Songs : NSManagedObject

@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSNumber * length;
@property (nonatomic, retain) Album *owner;

@end

假设我创建了 10 张专辑，每张专辑都有一些随机（正）歌曲。现在我想查找并获取歌曲数量最多的前 5 张专辑。

如何使用 MagicalRecords 做到这一点？

目前，我是这样做的，

NSLog(@"%@",[Album MR_findAllSortedBy:@"albumID" ascending:YES withPredicate:[NSPredicate predicateWithFormat:@"@max.@count.songs"]]);

//NSLog(@"%@",[Album MR_findAllSortedBy:@"albumID" ascending:YES]); //works fine

但它崩溃并显示以下消息：

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse the format string "@max.@count.songs"'

我的谓词字符串无效吗？如果是的话，实现这个的正确方法是什么。

Answer 1

根据 this post，您不能在提取时按计数排序。您需要获取所有项目，而不是对它们进行排序和 trim：

 NSArray* array = [Album MR_findAll];
 array = [array sortedArrayUsingDescriptors:@[ [NSSortDescriptor sortDescriptorWithKey:@"songs.@count" ascending:NO] ] ];
 array = [array subarrayWithRange:NSMakeRange(0, MIN( array.count, 5) )];

如何使用 MagicalRecord 过滤和获取记录？

How to filter & fetch records with MagicalRecord?

core-data

ios

magicalrecord