ios - 无法从核心数据中获取不同的属性值

Question

下面有 2 个函数，但只有一个有结果。在“noResult”中，我试图获取不同属性值的列表，但我得到的只是 {}。

-(void)noResult {
  NSManagedObjectContext *moc= [xmppMessageArchivingStorage mainThreadManagedObjectContext];
  NSEntityDescription *entity= [NSEntityDescription entityForName:@"XMPPMessageArchiving_Message_CoreDataObject" inManagedObjectContext:moc];
  NSFetchRequest *request= [[NSFetchRequest alloc] init];
  [request setEntity:entity];
  [request setResultType:NSDictionaryResultType];
  [request setReturnsDistinctResults:YES];
  [request setPropertiesToFetch:[NSArray arrayWithObject:@"contact"]];

  NSArray *messages= [moc executeFetchRequest:request error:nil];
  NSLog(@"%@", messages); 
}

-(void)gotResult {
   NSManagedObjectContext *moc= [xmppMessageArchivingStorage mainThreadManagedObjectContext];
   NSEntityDescription *entity= [NSEntityDescription entityForName:@"XMPPMessageArchiving_Message_CoreDataObject" inManagedObjectContext:moc];
   NSFetchRequest *request= [[NSFetchRequest alloc] init];
   [request setEntity:entity];
   NSArray *messages= [moc executeFetchRequest:request error:nil];
}

以下来自“gotResult”"(entity: XMPPMessageArchiving_Message_CoreDataObject; id: 0x1c5d7fc0 ; data: {\n bareJid = nil;\n bareJidStr = nil;\n body =\"Bean bags\";\n composing = 0;\n contact = 1000743469142;\n from = 7375011;\n message = nil;\n messageStr = nil;\n outgoing = 0;\n streamBareJidStr = nil;\n thread = nil;\n timestamp =\"2013-06-05 10:56:38 +0000\";\n})"

Answer 1

我能做出不同行为的唯一原因想到的是您在调用使用 NSDictionaryResultType 获取请求。

[request setResultType:NSDictionaryResultType];

暗示

[request setIncludesPendingChanges:NO];

这意味着从获取中返回的数组反射(reflect)了持久存储中的当前状态，并且不考虑上下文中任何未决的更改、插入或删除。