CoreData：获取与NSDictionaryResultType的一对多关系的计数 [英] CoreData: Fetch count of to-many relationship with NSDictionaryResultType

问题描述

我有一个大的对象列表在Core Data（约50 000和周期性增加）。我使用以下请求获取它：

I have a big list of objects in Core Data (about 50 000 and periodically increasing). I fetch it with the following request:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:[SongObject name]];
fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES]];
fetchRequest.propertiesToFetch = @[@"uid", @"name", @"toArtistRef.uid", @"toArtistRef.name"];
fetchRequest.resultType = NSDictionaryResultType;

实体SongObject也包含关系 toSongsInPlaylistRef 是对多。

Also entity SongObject contains relationship toSongsInPlaylistRef, which is to-many.

我需要为每个对象获取此集合的计数。由于大量的数据，我不能获取关系本身。
我尝试添加
@toSongsInPlaylistRef。@ count和 @toSongsInPlaylistRef.count
，但它崩溃

I need to fetch count of this set for each object. Due to big amount of data, I can't fetch the relationship itself. I tried adding @"toSongsInPlaylistRef.@count" and @"toSongsInPlaylistRef.count" but it crashes

Terminating app due to uncaught exception 'NSInvalidArgumentException', 
reason: 'Invalid to many relationship in setPropertiesToFetch: (toSongsInPlaylistRef.count)'

请写任何建议。

Please write any suggestions.

推荐答案

要获取相关对象的计数，必须创建一个 NSExpressionDescription
并包括在 propertiesToFetch ：

To get the count of related object, you have to create a NSExpressionDescription and include that in the propertiesToFetch:

NSExpression *countExpression = [NSExpression expressionForFunction:@"count:"
        arguments:@[[NSExpression expressionForKeyPath:@"toSongsInPlaylistRef"]]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName:@"playListCount"]; // Choose an appropriate name here!
[expressionDescription setExpression:countExpression];
[expressionDescription setExpressionResultType:NSInteger32AttributeType];

fetchRequest.propertiesToFetch = @[expressionDescription, ...];

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