将排序描述符应用于从模板创建的NSFetchRequest [英] Applying sort descriptor to NSFetchRequest created from template

问题描述

我在我的核心数据模型中定义了一个称为RemainingGaneProjections的获取请求。我想执行该获取请求并按照实体的一个属性对结果排序。我的代码看起来像这样：

I have a fetch request defined within my core data model called "RemainingGaneProjections". I want to execute that fetch request and sort the results by one of the entity's attributes. My code looks like this:

NSFetchRequest *projectionsRequest = [model fetchRequestTemplateForName:@"RemainingGameProjections"];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"confidence" ascending:NO];
[projectionsRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];

当我尝试执行此代码时，它会与以下消息崩溃：

When I try to execute this code it crashes with the following message:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Can't modify a named fetch request in an immutable model.'

我在调试器中确认，当我在NSFetchRequest上执行setSortDescriptors方法时，发生此崩溃。我还未能找出为什么会发生这种情况。

I have confirmed in the debugger that this crash happens when I execute the setSortDescriptors method on my NSFetchRequest. I haven't been able to figure out why this happens.

这里发生了什么？在检索需要排序的数据时，应该使用另一种方法吗？

Any explanations for what is happening here? Is there another approach I should be using when retrieving data that needs to be sorted?

推荐答案

记录所有地方。因为我的读取请求没有替换参数，我使用fetchRequestTemplateForName方法，而不是fetchRequestFromTemplateWithName。事实证明，Core Data编程指南说明：

I found the answer myself in the Apple documentation of all places. Because my fetch request has no substitution parameters, I used the fetchRequestTemplateForName method instead of fetchRequestFromTemplateWithName. As it turns out, the Core Data programming guide says this:

如果模板没有替换变量，则必须： p>

If the template does not have substitution variables, you must either:

1. 使用fetchRequestFromTemplateWithName：substitutionVariables：并传递
   nil作为变量参数;


2. 使用fetchRequestTemplateForName：和
   复制结果。如果您尝试使用
   fetchRequestTemplateForName：返回的获取请求，则会生成一个异常（不能
   在不可变模型中修改命名的提取请求）。

我修改了我的抓取请求初始化操作：

I modified my fetch request initialization to do this:

NSFetchRequest *projectionsRequest = [[model fetchRequestTemplateForName:@"RemainingGameProjections"] copy];

现在一切正常。

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