iOS：如何获取核心数据中数量最多的对象？

Question

我的核心数据模型中有两个类，它们是一对多的关系。

我们以班级专辑>班级歌曲为例进行说明。现在我想从Album 中获取（顶部）五个专辑，其中有Songs 的最大数量。

Class Album
{
    albumID (Integer)
    name    (String)
    songs   (class B)
}

Class Songs
{
    name   (String)
    length (Integer)
}

---

@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

---

这就是我正在尝试的，

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Albums" inManagedObjectContext:[self context]]];
fetchRequest.fetchOffset = 0;
fetchRequest.fetchLimit = 5;
fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"albumID" ascending:YES]];
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"SELF @max.@count.songs"];
NSError *error = nil;
NSArray *albums = [[self context] executeFetchRequest:fetchRequest error:&error];

如果我不通过谓词，它可以正常工作。但问题在于谓词。它使应用程序崩溃并显示以下消息：

*** 由于未捕获的异常“NSInvalidArgumentException”而终止应用程序，原因：“无法解析格式 字符串 "SELF @max.@count.songs"'

更新：我正在尝试与NSExpression 相同但无法同时执行max: 和count: 函数？以下是我尝试过的，

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Albums" inManagedObjectContext:[self context]]];

fetchRequest.fetchOffset = 0;
fetchRequest.fetchLimit = 5;

NSError *error = nil;

NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"songs"];
NSExpression *countExpression = [NSExpression expressionForFunction:@"count:" arguments:[NSArray arrayWithObject:keyPathExpression]];
//NSExpression *maxExpression = [NSExpression expressionForFunction:@"max:" arguments:[NSArray arrayWithObject:keyPathExpression]];

NSExpressionDescription *maxCountExpressionDescription = [[NSExpressionDescription alloc] init];
[maxCountExpressionDescription setName:@"squishes"];
[maxCountExpressionDescription setExpression:countExpression];

[fetchRequest setPropertiesToFetch:[NSArray arrayWithObject:maxCountExpressionDescription]];
[fetchRequest setResultType:NSDictionaryResultType];

NSArray *albums = [[self context] executeFetchRequest:fetchRequest error:&error];

我想，我需要这样查询，

从专辑中选择专辑名称，最大值（计数（专辑.歌曲））；

如何在NSFetchRequest中创建这样的查询，NSExpression或NSPredicate可以吗？

Answer 1

尝试删除：

fetchRequest.predicate = [NSPredicate predicateWithFormat:@"SELF @max.@count.songs"];

并将您的排序描述符替换为：

fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"songs.@count" ascending:NO]];

Answer 2

我以不同的方式解决了这个问题，我在 Album 类中添加了一个 songsCount 属性。每次从专辑中添加/删除歌曲时，我都会增加/减少它。然后我按降序获取所有按songsCount 排序的专辑。

一旦我可以获取所有专辑，我将运行一个 for 循环并取出前五个对象。

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Albums" inManagedObjectContext:[self context]]];
fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"songsCount" ascending:NO]];
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"songsCount > 0"];
NSError *error = nil;
NSArray *albums = [[self context] executeFetchRequest:fetchRequest error:&error];
NSMutableArray *top5Albums = [NSMutableArray array];
NSInteger totalCounts = [albums count];
NSInteger maxIndex = (totalCounts >= 5) ? 5 : totalCounts;
for(NSInteger index = 0; index < maxIndex; index++) {
    [top5Albums addObject:[albums objectAtIndex:index]];
}