NSFetchRequest setPropertiesToGroupBy：返回所有匹配的实体？ [英] Does NSFetchRequest setPropertiesToGroupBy: return all entities that match?

问题描述

我正在为iOS开发一个应用程序，我找不到有关新的 NSFetchRequest setPropertiesToGroupBy：方法的文档。

我有一堆与多个实体B对象有关系的实体A对象。

我想获取所有与特定对象匹配的实体B对象模式，并由实体A将它们分组。这是为了我没有为每个实体A单独的DB提取。结果将理想地被结构化，使得我可以迭代相等的实体As的组，然后将具有所有实体B匹配来自一个获取请求的模式。

我可以看到 setPropertiesToGroupBy：方法应该让我通过逆B-> A关系，并返回 NSDictionaries 的数组。但是这是否意味着 NSArray 中的每个项目对应于实体A的一个组？并且相应的 NSManagedObject 子类可以在 NSDictionary ？

< class =h2_lin>解决方案

没有。有两种方法可以执行此操作：

• 使用多个请求


• A关系为A→B关系的实体。然后包括 NSSortDescriptor ，以按B排序，并将结果分成组。

后者可能更快，因为内存中分割可避免重复的DB访问和提取请求解析。

I'm developing an app for iOS, and I can't find much documentation on the new NSFetchRequest setPropertiesToGroupBy: method.

I have a bunch of entity A objects that have relationships with multiple entity B objects.

I want to fetch all entity B objects that match a specific pattern and group them by entity A. This is so that I don't have a separate DB fetch for every entity A. The results would ideally be structured so that I could iterate over groups of equal entity As, and then would have all the entity Bs matching the pattern from one fetch request.

I can see that the setPropertiesToGroupBy: method should let me group by the inverse B->A relationship, and returns an array of NSDictionaries. But does that mean each item in the NSArray corresponds to one group by entity A? And that the corresponding NSManagedObject subclasses can be found somewhere in the NSDictionary?

解决方案

No it doesn't. There are two ways to do this:

• Use multiple requests
• Have one request to get all A entities where the inverse A->B relationship is in a group of Bs. Then include an NSSortDescriptor to sort by B and separate the results in to groups.

The latter is probably quicker because an in-memory split avoids repeated DB accesses and fetch request parsing.

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