永久 NSManagedObjectID 不是那么永久吗? [英] Permanent NSManagedObjectID not so permanent?

问题描述

我在处理 CoreData 中的对象 ID 时遇到问题.为方便起见，我使用 MagicalRecord 并有 3 个上下文:一个私有队列工作上下文、一个用于 UI 的主队列上下文和工作上下文的父级，以及一个作为主上下文父级的私有队列保存上下文.

I'm having trouble dealing with object IDs in CoreData. I'm using MagicalRecord for convenience and have 3 contexts: a private queue working context, a main queue context for UI and parent to the working context, and a private queue saving context that is the parent of the main context.

我的目标是在工作上下文中创建一个对象，保存到持久存储，将它的 objectID URL 保存到 NSUserDefaults，然后能够稍后使用 objectID 提取该 MO.但是，我发现保存对象的永久 ID 后发生了变化.

My goal is to create an object in the working context, save to the persistent store, save it's objectID URL to NSUserDefaults, and then be able to pull out that MO using the objectID later on. However, what I'm finding is that after saving the permanent ID of the object is changing.

在下面的控制台输出中，您可以看到在我请求永久 ID 后，我得到的值是F474F6EE-A225-456B-92EF-AB1407336F15/CDBaseAccount/p1"但稍后当我列出 CD 中的所有对象时那里唯一的对象具有 IDF474F6EE-A225-456B-92EF-AB1407336F15/CDBaseAccount/p2".p1 vs p2，发生了什么?

In the console output below you can see that after I request the permanent ID the value I get back is "F474F6EE-A225-456B-92EF-AB1407336F15/CDBaseAccount/p1" but later when I list all the objects in CD the only object there has the ID "F474F6EE-A225-456B-92EF-AB1407336F15/CDBaseAccount/p2". p1 vs p2, what happened?

代码:

  NSManagedObjectContext *c = [NSManagedObjectContext MR_contextThatPushesChangesToDefaultContext];
  [c performBlockAndWait:^{

      NSArray *all = [CDBaseAccount MR_findAllInContext:c];
      NSLog(@"count: %d", all.count);
      NSLog(@"all accounts = %@", all);

      CDBaseAccount *a = [CDBaseAccount MR_createInContext:c];
      a.accountName = @"foo";

      [c MR_saveNestedContexts];

      NSLog(@"temp a.objectID = %@", a.objectID);

      NSError *error;
      if (![c obtainPermanentIDsForObjects:@[a] error:&error]) {
          NSLog(@"perm id error: %@", error);
          return;
      }

      NSLog(@"perm a.objectID = %@", a.objectID);

      NSURL *u = a.objectID.URIRepresentation;

      dispatch_async(dispatch_get_main_queue(), ^{
          NSManagedObjectContext *d = [NSManagedObjectContext MR_defaultContext];

          NSArray *all = [CDBaseAccount MR_findAllInContext:d];
          NSLog(@"count: %d", all.count);
          NSLog(@"all accounts = %@", all);

          NSManagedObjectID *i = [d.persistentStoreCoordinator managedObjectIDForURIRepresentation:u];
          NSError *objWithIdError = nil;
          NSManagedObject *o = [d existingObjectWithID:i error:&objWithIdError];
          if (objWithIdError != nil) {
              NSLog(@"existing object error: %@", objWithIdError);
              return;
          }

          NSLog(@"o = %@", o);
          NSLog(@"o.objectID = %@", o.objectID);

      });
  }];

控制台输出:

  > +[NSManagedObjectContext(MagicalRecord) MR_contextWithStoreCoordinator:](0xa7c9b0) -> Created <NSManagedObjectContext: 0x83522a0>:  Context *** MAIN THREAD ***
  > count: 0
  > all accounts = (
  > )
  > -[NSManagedObjectContext(MagicalSaves) MR_saveWithErrorCallback:](0x8353de0) -> Saving <NSManagedObjectContext: 0x8353de0>:  Context *** MAIN THREAD ***
  > -[NSManagedObjectContext(MagicalSaves) MR_saveWithErrorCallback:](0x8195450) -> Saving <NSManagedObjectContext: 0x8195450>: *** DEFAULT *** Context *** MAIN THREAD ***
  > -[NSManagedObjectContext(MagicalSaves) MR_saveWithErrorCallback:](0x83522a0) -> Saving <NSManagedObjectContext: 0x83522a0>: *** BACKGROUND SAVE *** Context *** MAIN THREAD ***
  > temp a.objectID = 0x8187ee0 <x-coredata:///CDBaseAccount/tF392AC6A-3539-4F39-AC53-35F9E5B3C9322>
  > perm a.objectID = 0x8355800 <x-coredata://F474F6EE-A225-456B-92EF-AB1407336F15/CDBaseAccount/p2>
  > count: 1
  > all accounts = (
      "<CDBaseAccount: 0x844ca60> (entity: CDBaseAccount; id: 0x844a4c0 <x-coredata://F474F6EE-A225-456B-92EF-AB1407336F15/CDBaseAccount/p1> ; data: <fault>)"
  )
  > existing object error: Error Domain=NSCocoaErrorDomain Code=133000 "The operation couldn’t be completed. (Cocoa error 133000.)" UserInfo=0x864d8c0 {NSAffectedObjectsErrorKey=(
      "<CDBaseAccount: 0x864b8c0> (entity: CDBaseAccount; id: 0x86405c0 <x-coredata://F474F6EE-A225-456B-92EF-AB1407336F15/CDBaseAccount/p2> ; data: <fault>)"
  )}

推荐答案

简而言之，不要那样做 :)

Short answer is, don't do that :)

-objectID 在应用程序启动之间不可靠.在以下条件下保证其唯一性和可靠性:

-objectID is not reliable between launches of your application. It is guaranteed to be unique and reliable under the following conditions:

1. 在应用程序的单个生命周期内
2. 以其原始对象形式(不是 URL 或 NSString 形式)

将 -objectID 视为要存储在持久性存储区之外的永久唯一标识符通常会让您失望.Core Data 在对象的生命周期中多次更改 -objectID 的底层细节.

Treating the -objectID as a permanent unique identifier to be stored outside of the persistent store is going to fail you fairly often. Core Data changes the underlying details of the -objectID many times during the life of the object.

如果您需要一个外部可靠的独特产品，那么您需要自己创建一个.我通常建议将 [[NSProcessInfo processInfo] globalUniqueString] 添加到任何需要外部引用唯一的实体.-awakeFromInsert 是一个很好的地方.

If you need an externally reliable unique then you need to create one yourself. I generally recommend adding a [[NSProcessInfo processInfo] globallyUniqueString] to any entity that needs an externally reference-able unique. -awakeFromInsert is a great place to do that.

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