核心数据NSBatchDeleteRequest似乎将对象留在上下文中 [英] Core Data NSBatchDeleteRequest appears to leave objects in context

问题描述

我已经看到许多有关Core Data中批量删除的问题，但是似乎没有一个问题可以解决我的问题。

我正在使用Core Data创建iOS 9 / Swift应用。在今年的WWDC上，我参加了Core Data会议，发现可以使用 NSBatchDeleteRequest 直接从持久性存储中删除大量对象。这适用于某些对象，但不适用于其他对象，我认为这与我的关系有关。

我有一个包含主题和课程，那里存在一对多的关系。主题可能拥有他们想要的任意多的课程。

主题有一个课程关系，但有一个删除级联的规则，因为我希望在时删除与主题相关的所有课程主题被删除。

在 Course 上逆是'subject'，带有一个删除 Nullify 的规则。在这里，我对Apple对 Nullify 的描述感到有些困惑：

删除对象之间的关系，但不要删除任何一个对象。
仅当雇员的部门关系是可选的，或者您确保在下一次保存操作之前为每个雇员设置一个新部门时才有意义。

这很清楚，但是为什么要删除关系但不能删除任何一个对象？如果删除 Course ，我希望删除 Course 并删除课程进行主题，以便删除的 Course 出现故障不会出现在主题的课程设置的 NSSet 中

我想提供一种删除实体中所有对象的方法。当我通过分别获取和删除每个课程进行尝试时，正确删除了个课程并将其从<$ c $中删除c>在主题上的个课程的NSSet 。

由于我不知道会有多少个课程，并且我想确保在每种情况下都具有较高的性能，因此我认为我将使用批量删除来删除所有课程。问题是，在利用 NSBatchDeleteRequest 删除所有主题时，可以正常工作，删除所有课程（由于 Cascade 规则），试图使用此方法删除所有课程方法似乎将所有对象保留在原处。

我使用了 NSBatchDeleteRequest 删除了所有课程，但是当我查询 MOC 来查看主题和课程仍然存在，课程仍然返回，并且主题仍然拥有它们

相反，当我分别获取并删除每个 Course 时，随后的获取均会正确显示所有课程的空数组，并且主题上的'课程'关系似乎已被正确修改。

是的，我在执行请求后保存了上下文。我想可能不会通知上下文商店的工作，但是再次删除所有主题效果很好。

解决方案

在描述 NSBatchDeleteRequest 的WWDC 2015会话，解释为更改未反映在上下文中 。因此，您看到的是正常现象。批处理更新直接在永久性存储文件上运行，而不是通过托管对象上下文进行操作，因此上下文不了解它们。通过获取然后删除来删除对象时，您正在浏览上下文，因此它知道您正在进行的更改（实际上，它正在为您执行这些更改）。

如果使用 NSBatchDeleteResultTypeObjectIDs ，则可以使用 mergeChangesFromRemoteContextSave：intoContexts：更新您的上下文。如果您没有从上下文中加载任何其他托管对象，则可能还可以使用 reset 。

I have seen many questions regarding batch deletion in Core Data, but none seem to address my issue.

I am creating an iOS 9 / Swift app using Core Data. At WWDC this year, I attended the Core Data session and saw that I could use NSBatchDeleteRequest to delete large numbers of objects directly from the persistent store. This works for me for some objects but not others, and I think it has something to do with my relationships.

I have an object graph consisting of Subject and Course, where there is a one-to-many relationship. Subjects may own as many courses as they wish.

There is a 'courses' relationship on Subject with a delete rule of Cascade, as I want all courses associated with a subject to be deleted when a subject is deleted.

The inverse is 'subject' on Course, with a delete rule of Nullify. Here, I am a bit confused as to Apple's description of Nullify:

Remove the relationship between the objects but do not delete either object. This only makes sense if the department relationship for an employee is optional, or if you ensure that you set a new department for each of the employees before the next save operation.

That makes it pretty clear, but why would the relationships be deleted but not either object? If I delete a Course, I would like the Course to be deleted and the relationship from the subject to the course to be deleted so that a fault to the deleted Course will not appear in the NSSet on Subject's courses Set.

I want to provide a way for all objects in an entity to be deleted. When I try this by individually fetching and deleting each course, courses are properly deleted and removed from the NSSet of courses on a Subject.

Since I have no idea how many courses will be present and I want to ensure high performance in every situation, I figured I would use batch deletion to delete all courses. The problem is that while utilizing NSBatchDeleteRequest to delete all Subjects works fine, deleting all courses along the way (because of the Cascade rule), trying to delete all Courses using this method appears to leave all objects in place.

I used NSBatchDeleteRequest to delete all Courses, but then when I query the MOC to see what Subjects and Courses still exist, both Courses are still returned and the Subject owning them still has references to them.

In contrast, when I fetch and delete each Course individually, my subsequent fetch properly displays an empty array for all Courses and the 'courses' relationship on the Subject appears to have been properly modified.

Yes, I am saving the context after executing the request. I suppose the context may not be notified of what the store does, but then again deleting all subjects worked great. What is going on here?

解决方案

At the WWDC 2015 session which describes NSBatchDeleteRequest it was explained that "Changes are not reflected in the context". So what you're seeing is normal. Batch updates work directly on the persistent store file instead of going through the managed object context, so the context doesn't know about them. When you delete the objects by fetching and then deleting, you're working through the context, so it knows about the changes you're making (in fact it's performing those changes for you).

If you use NSBatchDeleteResultTypeObjectIDs, you can merge the results of the batch delete back into your context using mergeChangesFromRemoteContextSave:intoContexts: to update your contexts. You could probably also use reset, if you don't have any other managed objects loaded from the context.

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