NSSortDescriptor具有一对多关系 [英] NSSortDescriptor with to-many relationship

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本文介绍了NSSortDescriptor具有一对多关系的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!

问题描述

我有一个父实体,它有许多孩子。如果我使用属性(例如名称)订购父提取,一切工作正常。但是如果我尝试这样:

  NSSortDescriptor * sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@child。@ count没有]; 

它不工作。我也试图添加

  [fetchRequest setRelationshipKeyPathsForPrefetching:[NSArray arrayWithObject:[entityProperties objectForKey:@child]]] ; 

但是效果不好。


解决方法:我需要添加一个属性并在每次添加一个孩子时更新它。方案

假设您使用的是sqlite存储;您不能按 @count 排序,因为它在对象级别解析,而 NSSortDescriptor NSFetchRequest 在数据库级别解析。因此,你猜到,你需要创建一个计数属性来排序。



如果你使用二进制或内存存储类型,则count应该工作得很好因为它们只能作为对象访问。


I have a Parent entity which has to-many Child. If I order the Parent fetch with a property (e.g. name), everything works fine. But if I try this:

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"child.@count" ascending:NO];

It doesn't work. I've also tried to add

[fetchRequest setRelationshipKeyPathsForPrefetching:[NSArray arrayWithObject:[entityProperties objectForKey:@"child"]]];

But it didn't work as well.

Am I allowed to sort by to-many.@count or do I need to add a property and update it each time I add a child?

解决方案

Assuming you are using a sqlite store; you cannot sort by @count because it is resolved at the "object" level whereas the NSSortDescriptor on the NSFetchRequest is resolved at the database level. Therefore, as you guessed, you would need to create a counting property to sort by.

If you are using a binary or in-memory store type then count should work fine since they are only accessed as objects.

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