永久NSManagedObjectID不是那么永久？

Question

我在处理CoreData中的对象ID时遇到问题.我正在使用MagicalRecord以方便并且有3个上下文:私有队列工作上下文,UI的主队列上下文和工作上下文的父级,以及作为主上下文的父级的私有队列保存上下文.

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,然后能够使用objectID拉出该MO.但是,我发现在保存对象的永久ID后正在改变.

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,唯一的对象就在那里ID为"F474F6EE-A225-456B-92EF-AB1407336F15/CDBaseAccount/p2".p1 vs p2,发生了什么？

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>)"
  )}

Answer 1

简短的回答是,不要那样做:)

-objectID在您的应用程序启动之间不可靠.在以下条件下,它保证唯一且可靠:

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

将-objectID持久性唯一标识符作为永久性唯一标识符存储在持久性存储之外将会经常失败.核心数据-objectID在对象的生命周期中多次更改底层细节.

如果您需要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[[NSProcessInfo processInfo] globallyUniqueString]在需要外部可引用唯一的实体中添加a . -awakeFromInsert是一个很棒的地方.