使用 Core Data 获取会生成“无法识别的选择器错误”？
我正在尝试检索使用核心数据保存的对象列表。创建项目时 Xcode 的默认设置没有发生任何更改。实际数据存储中存在项目，并且实体 Transaction 在保存时工作正常，但在运行以下代码时：
NSManagedObjectContext * context = [[NSApp delegate] managedObjectModel];
NSFetchRequest * fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription * entity = [NSEntityDescription
entityForName:@"Transaction"
inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSError * error = nil;
NSArray * transactionList = [context executeFetchRequest:fetchRequest error:&error];
if (&error != nil) {
[Utility showMessage:error.description asError:YES];
} else {
[Utility showMessage:[NSString stringWithFormat: @"Items: %@", transactionList.count] asError:NO];
}
尝试创建 entity 对象时收到以下错误。
[NSManagedObjectModel permanentStoreCoordinator]：发送到实例的无法识别的选择器
我缺少什么，或者我该怎么做才能检查导致错误的原因？
脚注
- Utility 是一个静态类，它只是生成一个 NSAlert 框。
- 我一直在使用这个教程来尝试理解如何代码有效
I'm trying to retrieve a list of objects saved using Core Data. No changes where made to the default setup made by Xcode when creating the project. There are items in the actual data store, and the entity Transaction works fine when saving but when running the following code:
NSManagedObjectContext * context = [[NSApp delegate] managedObjectModel];
NSFetchRequest * fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription * entity = [NSEntityDescription
entityForName:@"Transaction"
inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSError * error = nil;
NSArray * transactionList = [context executeFetchRequest:fetchRequest error:&error];
if (&error != nil) {
[Utility showMessage:error.description asError:YES];
} else {
[Utility showMessage:[NSString stringWithFormat: @"Items: %@", transactionList.count] asError:NO];
}
I receive the following error when trying to create the entity object.
[NSManagedObjectModel persistentStoreCoordinator]: unrecognized selector sent to instance
What am I missing, or what do I do to check what is causing the error?
Footnotes
- Utility is a static class which simply generates a NSAlert box.
- I've been using this tutorial to try and understand how the code works
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评论（2）
在第一行中，您从应用委托中获取
managedObjectModel并将其分配给NSManagedObjectContext。您应该改为获取managedObjectContext。In the first line you're fetching the
managedObjectModelfrom your app delegate and assigning it to anNSManagedObjectContext. You should fetch themanagedObjectContextinstead.从您的代码中，尚不清楚您到底要分配给托管对象上下文的内容。它应该是托管对象上下文，而不是托管对象模型。
另外，您应该检查
(error!=nil)是否不是&error。阅读 C 指针语法 (;-)。From your code, it is not clear what exactly you are assigning to managed object context. It should be a managed object context, not a managed object model.
Also, you should check if
(error!=nil)not&error. Read up on your C pointer syntax (;-).