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

问题描述

我在 Core Data 中有一个很大的对象列表(大约 50 000 个并且定期增加).我通过以下请求获取它:

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;

还有实体 SongObject 包含关系 toSongsInPlaylistRef，这是对多的.

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

我需要为每个对象获取这个集合的计数.由于数据量很大，我无法获取关系本身.我尝试添加@"toSongsInPlaylistRef.@count" 和 @"toSongsInPlaylistRef.count"但它崩溃了

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)'

请写下任何建议.

推荐答案

要获取相关对象的数量，您必须创建一个 NSExpressionDescription并将其包含在 propertiesToFetch 中:

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, ...];

这篇关于CoreData:使用 NSDictionaryResultType 获取对多关系的计数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！