核心数据：在一对多关系中按计数排序 [英] Core Data: sorting by count in a to-many relationship

问题描述

我目前正在尝试设置一个NSFetchedResultsController，它将基于一对多关系中的实体数量来排序我的表视图。我不知道这是否会在计数时产生差异，但这也是一个反比关系。

I am currently trying to setup a NSFetchedResultsController that will order my table view based on the number of entities in a to-many relationship. I am not sure if this makes a difference when counting, but this is also an inverse relationship.

我以为这样的工作很好：

I thought something like this would work just fine:

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Parent"
inManagedObjectContext:managedObjectContext];

NSSortDescriptor *sortDescriptor1 = [[NSSortDescriptor alloc] 
initWithKey:@"children.@count" ascending:YES];

NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor1,
nil];

[fetchRequest setSortDescriptors:sortDescriptors];

我不断收到包含KVC集合的Keypath，未能处理孩子。@ count'。

I keep getting 'Keypath containing KVC aggregate where there shouldn't be one; failed to handle children.@count'.

对可能出错的任何想法？

Any ideas on what could be going wrong?

推荐答案

据我所知，你不能在查询中应用@count，但是你可以很容易地将它应用到抓取的数组。

AS far as I know you can't apply the @count in the query, but you can easily apply it to the fetched array.

NSEntityDescription * entity = [NSEntityDescription entityForName:@"Parent" inManagedObjectContext:self.managedObjectContext];

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];
NSError *error;
NSArray *results = [self.managedObjectContext executeFetchRequest:request error:&error];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"children.@count" ascending:NO];
NSArray *descriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
NSArray *sortedArray  = [results sortedArrayUsingDescriptors:descriptors];

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